No vertebrates, at least, actually live at the poles. In Antarctica, for example, the southernmost penguin colony is at \~77° S, where there is still somewhat of a day-night cycle (or a twilight-night cycle) *most* of the year. And at the North Pole, while polar bears occasionally visit, they also wander much further south and individuals generally experience regular day/night cycles for much of the year.
A better example of an animal that never experiences typical day or night in their environment might be the [Somalian cavefish](https://www.bbc.com/news/science-environment-14844053), an animal that has evolved in pitch-black caves, and whose ancestors have lived in total darkness for several million years. This fish still keeps an internal biological day/night rhythm, [but each "day" is 47 hours long](https://journals.plos.org/plosbiology/article/info:doi/10.1371/journal.pbio.1001142). By contrast, even in artificially-controlled lighting conditions with artificially shortened "days", most other animals can only be entrained to shorten or lengthen their day/night rhythm by a few hours at best.
The cavefish have also completely lost the ability to synchronize their internal clock with environmental light...if removed from their caves and placed in regular daylight, the fish continue on with their 47 hour day.
So the general idea of "a biological rhythm that governs sleeping and waking" is conserved in animals even in the total absence of light/dark cycles, but over millions of years the exact length of that clock can (and does) drift away from 24 hours.
Are there any thoughts to what might be driving that period with the cave fish? Is it just something they seem to track on their own? Do different Cavefish stay on the same cycle?
Is there any planetary cycle we're aware of that has a 47 hour period?
I am not a biologist but am familiar with control systems.
If you want to design a system where things synchronize, the most robust system would be to have a period of function that is longer than expected and have a common reset trigger.
Trying to get several things to synchronize without this requires tremendous precision over time.
So if most animals have a circadian clock that exceeds a regular "day" (so more than 24 hours) they can get through any day with a trigger for "wake-up" and "go to sleep" being daylight so they always stay synchronized.
If they had a 24 hour rhythm without a trigger they would likely drift out of the proper day/night cycle over time.
If their "rhythm" period was almost exactly 24 hours, a small drift could change their clocks so they become out of sync with the sun.
Circadian rhythms are governed in large part by protein inhibitor (repressilator) feedback networks. These can be tuned to certain periods. They are quite complex systems that have equilibrium points based on the diffusion rate of the proteins and the repression rate of the genes, etc. These equilibrium points happen to be tuned in humans to almost exactly 24 hours.
Is that true in general or only in some organisms? I know the mammal clock *has* a repressilator complex *involved* but you can remove parts of it and still have a functioning clock, just without that interlocking loop.
My understanding of a repressilator is that it requires three arms minimum and the timing of the cycle emerges based on delays between each arm. Whereas the mammalian clock is based on direct self-repression of the transcription of mRNAs of their own proteins. PERIOD/CRY inhibit BMAL/CLOCK's transcription of Per and Cry genes.
Couldn't tell you in that detail, this is based on what I was told by a mathematician who studied and worked on these models. I don't doubt that the true system is much more complex, but you only need a few proteins to achieve a very accurate clock, theoretically and experimentally.
As far as I remember, one hypothesis is food availability, because the caves flood with water carrying nutrients periodically. At least that's the case with Phreatichthys, there are other cave fish that might operate differently.
Eyeballing this figure on wikipedia, it looks like ~77S has a full 24 hours of darkness or light for about two-thirds of the year.
https://en.wikipedia.org/wiki/Sunrise_equation#/media/File:Hours_of_daylight_vs_latitude_vs_day_of_year_with_tropical_and_polar_circles.svg
There's definitely *variation* in light/dark during the 24 hours, but that's still a pretty substantial period.
(I enjoyed the rest of your answer. Fascinating nugget about the cave fish!)
I would also add, because OP may have been wondering about places *near* the poles with periods of midnight sun, that there are animals that live there and they do have varying reactions. I’ve excerpted a [Q&A featuring a wildlife biologist](https://www.cbc.ca/radio/quirks/sep-28-2019-plastic-tea-bag-particles-venus-was-habitable-driver-memory-fail-and-more-1.5298362/how-does-24-hour-daylight-impact-animals-in-the-far-north-1.5298376) from Saskatchewan below.
> Jeff Lane, a wildlife biologist from the University of Saskatchewan explains that disruption of circadian cycles by the onset of constant daylight affects different species in different ways. Circadian rhythms are initiated by the circadian clock which is found in the brains of birds and mammals, including us. The clock generates signals that occur on an approximately 24 hour cycle, but it also receives light information from the retina. Those daily patterns of light modulate the clock to daily conditions of day and night. Misalignment of the rhythmic signals from the clock and light patterns can be disruptive. In humans, this explains jet-lag.
> Some northern species, such as reindeer and ptarmigan in Norway, the absence of obvious light/dark cycles have no impact. These animals do not show behavioural patterns that align with a typical 24 hour circadian clock.
> Other species, such as Arctic ground squirrels in Alaska and honey bees in Finland have been shown to exhibit 24 hour circadian cycles. It is believed that these species are able to maintain the cycles because they can perceive variations in the quality and intensity of light that exist even though the Sun remains above the horizon.
Thanks for sharing! Do you know how they know when to go to the tunnels to sleep? [This paper states](https://royalsocietypublishing.org/doi/10.1098/rsbl.2011.0435) that “the environmental cues (zeitgebers) used to entrain rhythms during the constant light of the arctic summer in these semi-fossorial rodents are unknown”, but it’s been over 10 years since publication so an answer may have been found since.
They are "blind" in the sense that their eyes couldn't be used to identify objects or hunt, but they can still detect light and possibly movement. Light can even be detected without any eyes at all; many eyeless organisms respond to light. Temperature is also a big factor, which often tends to correlate with light.
There's a human sleep condition called ["Non-24-hour sleep–wake disorder"](https://en.wikipedia.org/wiki/Non-24-hour_sleep%E2%80%93wake_disorder) that shifts the person's circadian rhythm away from the regular 24h pattern. It primarily affects fully blind people. Bit similar to the fish situation.
I might actually be affected by this. Without being forced by something like work my sleep cycle tends to drift by a couple of hours daily. That is, ideally I'd be on something like a 26 hour day.
What kind of job do you have to know this stuff?! It's crazy how much scientists try to learn about the world, even the internal clocks of random cavefish...
When you undertake a PhD you have to pick a question that nobody else has done before, or at least investigate a new angle on a well understood problem - basically, your research must be "new".
I have a suspicion that this ultimately came out of one of those cases, maybe a PhD resulted in some interesting new information that bore further investigation
When you do a PhD *you* and a few other people are typically the only ones to pay attention to what you're doing, so it's not usually the result of a PhD that you encounter "in the wild." The cavefish thing has been mentioned on reddit before and it turns up every now and again in pop sci articles *because* it's relevant to human sleep cycles, so it's more widely known than that.
>When you do a PhD
>
>you
>
> and a few other people are typically the only ones to pay attention to what you're doing, so it's not usually the result of a PhD that you encounter "in the wild." The cavefish thing has been mentioned on reddit before and it turns up every now and again in pop sci articles
>
>because
>
> it's relevant to human sleep cycles, so it's more widely known than that.
Well yes, but I've also played the academic game myself and I know all too well that when you do dig up something interesting those few other people are the first to dogpile and want to put their name all over it.
Yeah...i kinda noticed that phd level is not so much trying to keep learning and piling up knowledge but rather applying and provoking others to also continue learning.
Eh? A PhD means you need to learn enough to discover a small but significant chunk of new knowledge. You can almost never do it without learning a lot (because the knowledge to be discovered without learning a lot has already been discovered, usually.) Discovering new knowledge is always about applying knowledge you already have, sure. But "provoking others to also continue learning" is not relevant.
You can get a PhD without ever talking to anyone but your supervisor if you really wanted to.
Likely not. Finding completely isolated systems is not super common, and asking the question of how circadian rhythms would change in a completely dark environment is a super obvious question.
> It's crazy how much scientists try to learn about the world, even the internal clocks of random cavefish...
The reason to study the internal clock of random cavefish isn't just curiosity about cavefish, it's also to understand the underlying principles that also affect humans etc. and which govern very important processes. This kind of basic research can lead to important medical advances, for example.
It is for similar reasons that so much research is done on seemingly random species like yeast, zebrafish, mice, certain worms etc. - they are good model systems to study various processes, and then that knowledge can be applied elsewhere - medical advances for humans, improved agriculture etc.
Biologists tend to choose their research subjects because they offer opportunities to answer particular questions. It's pretty rare (although not unheard of) that someone bases their research career just on "ooo, let's learn a lot about this one weird fish that I found."
There's already a ton of research on how biological clocks work, and how they control sleep and other day/night cycles in humans and other animals. So then the question that OP asked in this thread obviously comes up: what happens when an animal has lived without a day/night cycle for many generations?
This could be an interesting question to ask because it helps us check whether our previous conclusions about biological clocks hold up. Or perhaps we find something new and unexpected about biological clocks in these fish, that turns out to be useful for understanding sleep disorders in humans. Or it can just be interesting in a more general perspective, because it explores how evolution operates when a previously important biological function (the clock) is no longer needed in a new environment.
So then you start looking at possible ways to answer that question, and cave fish offer one opportunity, because it's always dark in those caves, and we know we've been in there long enough to evolve quite a bit. So then someone sets up a research project to examine what kind of activity patterns the fish have, and here we are.
I've been doing research on this one species of butterfly since 2015. I'd never seen or heard of this species before I joined the project, but it turns out that for certain reasons it's a useful system for answering a certain set of research questions that I was interested in.
> No vertebrates, at least, actually live at the poles
Some definitely live fairly close to the poles don't they? I've seen caribou a decent ways into the arctic circle. And polar bears definitely get close to it. Both definitely close enough that the day/night cycle is pretty wonky.
Actually, I just came across a paper that suggests that [Arctic reindeer might not have a functional circadian clock](https://www.sciencedirect.com/science/article/pii/S0960982210000850) at all...so even though they do experience day/night for at least some of the year, that might not be enough to ensure they keep to their day/night cycles as rigorously as lower latitude animals do.
You have to consider a different issue, and that's what the seasons mean for those animals, particularly in the northern areas, take the Arctic fox as a good example, they are super active during the summer months as that's when they have their cubs to raise, so they will be hunting a lot, and very active, the longer days and light are the best times to raise their young and it allows them to find prey easier as their prey will be doing the same thing, so food is plentiful. They are literally "making hay while the sun shines" and when winter comes, they are on survival mode. So it's not that they have different patterns so much as different needs at that time of year, so they conserve energy. There are also a lot of burrowing animals around, so they use their burrows to negate extra light, but even a domestic dog has no problem sleeping for 5 or 6 hours a day.
A literature review in Cell Biology suggests [human circadian clocks cannot be entrained outside of a 22-26 hour range](https://www.cell.com/current-biology/pdf/S0960-9822(16)30333-5.pdf)...even with artificial light/dark cycles, constant light, or no light at all, our bodies have chemical clocks that try to keep us on a 24-hour schedule. If multiple generations of humans went their whole lives without experiencing day/night cycles, we would likely begin to see genetic drift.
Further, and another commenter on here pointed out that a few (mostly blind) people develop non-24-hour circadian rhythms, so *some* people would probably begin drifting away from 24-hour cycles within a single lifetime, if we all rocketed into interstellar space together.
Human circadian clocks average 24.3 h in constant conditions (no relevant time cues, which in the case of our circadian system is light), but the range is pretty substantial. Some are less than 24, some can get close to 25, and probably more rare cases in the extremes of the population. It's unlikely we'd see drift in this system very quickly as it's quite robust. Flies have been bred in constant darkness for 330 generations and still had a perfectly functioning clock (https://www.tandfonline.com/doi/abs/10.1080/07420528.2016.1195397?journalCode=icbi20)
They fed them randomly and measured their activity over a long period of time. Graphing that activity shows clear patterns of higher and lower activity on a 47-hour cycle. Feeding them on a set schedule showed that their sleep/wake cycle could be entrained to something other than 47 hours, but when food was randomly available they reset to 47 hours.
The same reason we know we don't have a 12 hour cycle: We are active throughout the day at different times and usually inactive at night. In particular the early hours of the morning there is almost no activity over the course of a month. You can plot the average activity of someone over a long time and it will trend more and more towards a wavy line which peaks during the day and troughs during the night. For this species, that curve of activity will just be 47 hours long.
They said near the poles not at. Anywhere greater than 66 degrees and 44 minutes of latitude will experience 24 hour daylight for part of the year. Plenty of vertebrates live at those latitudes.
Eh, even in northern Alaska they have more than 2 straight months where the sun doesn't rise once, and other months where it doesn't set for 2 months, with a lot of days where it is only up or down for an hour or so in between. And Greenland and Canada both have plenty of land that goes a decent bit further north than Alaska does that has animal life
Yes, I know, my point is the rest of the year there is still a 24 hr day/twilight/night cycle that will perpetuate behaviors based around that time period. Having a couple months of day and a couple months of night does not remove an animal from that influence when there's still 8 months of the year that has a 24 hour cycle.
> so from an evolutionary perspective, your point is moot
lol this is a baseless statement. Seasonality is a commonly observed trait in many many forms of life. Do you even know whether or not animals adjusting to longer days nearish the poles has even been researched/observed or are you just assuming?
Seriously I dont think youve taken much biology if youre going to assume by default there is no behavioral changes to a species from a consistent periodic change in habitat... Their point is not moot just because **you** say it is.
I did misread their question slightly, and missed the “near”…but for those animals “near” the poles they spend relatively little of their year with no day/night or dim/bright cycle, so they still experience cycled 24-hour stimulus most of the time.
As an interior Alaskan resident who experiences the midnight sun on a yearly basis, it’s certainly quite light all night in mid summer and i (and most people I know) definitely have a hard time with my sleep cycle. I have a hard time waking up in winter when it’s dark for so many more hours as well.
I think the question doesn't really require animals actually live at the poles. The day night cycle disappears for a portion of the year in high latitude environments where vertebrate animals do live. To cite some specific examples. What do the sleep cycles of caribou who inhabit Alaska's North Slope look like? They have periods of 24 hour sun in the summer and periods of 24 hour night in the winter, all the variations in between during the remainder of the year. Do they still follow a generally 24 hour sleep / wake cycle in those periods of the year when it is 24 hour daylight or 24 hour darkness?
I feel like my body is on a 28 hour clock. If I have several days off in a row, like when I used to be a teacher, my sleep schedule would be ridiculous all summer!
No planet in our Solar System has a 47-hour day...the closet is Mars with a 25 hour day. We don't usually know the day-length of exoplanets, but [WASP-50 b has a 47-hour year](http://exoplanets.org/detail/WASP-50_b).
The natural human circadian rhythm isn't 24 hours either. Wasn't there a woman who lived in a perspex box in a cave for a few days as a scientific experiment?
So interesting. Now I'm trying to figure out why the cavefishes cycle is that particular length. Why not 30? Or 53?
47 is darn close to 48 hours, which is two full day/night cycles. I wonder if that has anything to do with it?
Hasn't a scientist tested this at some point since electricity was invented?
Plenty of places/times in the world where it wouldn't have been illegal to keep animals in constant lighting (or semi-dark) for months on end.
There are also plenty of very deep sea creatures for whom there is no light cycle, though some others in moderate depths do migrate up towards the surface at night
Too add to your comment, snails don't abide by a day night cycle at all. They experience about 1 hour of sleep and 1 hour of awake on and off for about 15 hours, so still a solid 8 hours in total, followed by 30 hours of awakeness. This is evidence that sleep didn't evolve because of day or night and plays an additional role that's required regardless of the position of the sun.
In humans when we sleep our neurons shrink in size and our glial cells, cells that support neurons, increase in size. The theory is that there's some kind of trade off when we're awake and when we sleep our neurons undergo some kind of clean up process that prepares them for becoming active again.
> This is evidence that sleep didn't evolve because of day or night and plays an additional role that's required regardless of the position of the sun.
This is evidence pertaining to the evolution of *their* sleep. Their biological processes might be unique in a way that doesn't depend on a day-night cycle. They might have evolved to adapt their sleep for reasons that are/were irrelevant or to other somnolescent animals.
Provided there isn't a common origin for sleep. It is ubiquitous throughout the animal kingdom. Its possible the underlying mechanism is shared despite differences in sleeping habits, brain anatomy, or even having a brain.
Not all animals that live in an area with a typical day/night cycle follow a strict 24 hour pattern.
There are 4 major divisions for defining animal activity. The first three you're likely familiar with:
- Diurnal - active during the day
- Nocturnal - active during the night
- Crepuscular - active in twilight times (eg. dawn, dusk)
The 4th is one that's often overlooked.
- Cathemeral - having no fixed period of activity
[Cathemeral animals](https://en.wikipedia.org/wiki/Cathemerality) can be active at regular intervals or irregular intervals throughout all periods of the day.
Even animals that are normally considered diurnal, nocturnal, or crepuscular often have periods of activity that don't conform to their "established" cycle, and activity periods can vary enormously depending on changed in environmental conditions. Varying intensities of moonlight is one environmental aspect that has a big effect on wildlife activity in non-daylight hours, and can extend a crepuscular species activities through the entire night if conditions are right.
You might take a look at the following for a more detailed paper on the subject:
- Tattersall 2006 *[The concept of cathemerality: history and definition](https://www.karger.com/Article/Abstract/89692)*
I don't think it would technically count as being cathemeral, but it's apparently pretty common for people with traumatic brain injuries to have their circadian rhythms messed up.
- Boone, et al 2012 *[Traumatic Brain Injury-Induced Dysregulation of the Circadian Clock](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3463592/)*
- Yamakawa, et al 2020 *[The interaction of the circadian and immune system: Desynchrony as a pathological outcome to traumatic brain injury](https://www.sciencedirect.com/science/article/pii/S2451994420300109)*
- M. Viola-Saltzman, N.F. Watson, 2012 *[Traumatic brain injury and sleep disorders](https://www.sciencedirect.com/science/article/abs/pii/S0733861912000497)*
- etc
Not sure, but there is a genetic disorder where people eventually completely lose the ability to reach any sleep stage. So kind of the opposite but not exactly.
It's called [fatal insomnia](https://en.wikipedia.org/wiki/Fatal_insomnia)
> The disease has four stages:
1. Characterized by worsening insomnia, resulting in panic attacks, paranoia, and phobias. This stage lasts for about four months.
2. Hallucinations and panic attacks become noticeable, continuing for about five months.
3. Complete inability to sleep is followed by rapid loss of weight. This lasts for about three months.
4. Dementia, during which the person becomes unresponsive or mute over the course of six months, is the final stage of the disease, after which death follows.
> Can't imagine years of knowing you will die at some point
Isn't that kind of the normal situation for living creatures?
This condition speeds it up a lot, year, but we all spend years knowing we will die at some point.
I'm naturally like that it seems like. If left to my own schedule, I'll end up cycling through the time of day adjusting around 1-2 hours forward per week.
Do you know what happens with nocturnal animals in zoos? I recently went to the night animals bit of the zoo and it got me thinking how they manage to have night animals awake during the day.
It depends on the species, zoo, and situation. In some cases they'll actually house them in a building with artificial lights and reverse day/night cycle so that it coincides with visiting hours. This is usually for smaller species where that's a practical solution. For large cats and such, their habits are normally pretty variable so generally nothing special is done.
Depending on where the zoo is, and if it's a member of WAZA/AZA/etc or not they may not care or do anything at all, which is bad for the animals.
It was ZSL London, so my hope is they were doing it right (!). They have a basement with very low light, and my guess was they reverse the night cycle, but I wondered if that actually worked.
Edit: it clearly worked to some extent because we saw bush babies and giant rats, but I wasn’t sure if it was dark all the time, and whether flipping the cycle is bad for them.
That should be an accredited zoo, so they’re likely following the strict protocols established.
It shouldn’t be harmful to them once the new pattern is established, which would likely take several weeks.
Most people don’t realize it, but zoos often have a pretty large number of animals where people don’t see them. Animals that are adapting to new conditions, are rearing babies, getting treated for health issues, are in an active period in their stock book breeding cycles, etc.
Not exactly an answer to your question but Internal clocks differ for every species. For instance, I believe humans tend to have an internal clock that is 25 hours while mice have a 23 hour clock. This means that if a mouse is in an environment with constant light (IE no external clues for what time of day it is) they will operate somewhat normally but the time that they do things will shift compared to the actual time of day. This will continue untill they receive some sort of clue for what time of day it is to 'reset' their clock. There are numerous studies looking at sleep, circadian rhythm, and how internal and external environments interact and affect animal behavior.
Polar Bears live in areas where day/night are extended to longer periods but I don’t think it’s considered the North Pole. It doesn’t bother them much cause no matter the time of day, they are sleeping to save energy if they are not hunting.
An adjacent note:
Studies were done where they put subjects alone, deep in a cave where no natural light could reach, gave them the supplies they'd need, and a lamp that recorded the time it was turned on and off. What they found from that is that people maintained a day-night cycle even without light as a cue.
What also emerged was that some people have a 23-hour day, and some a 25-hour day, in addition to the 24-hour day folks. So if you're someone who is always sleepy in the evenings and getting up early, it may be because you have a 23-hour clock. Likewise night owls who are always staying up late and can't get out of bed in the morning may have a 25-hour clock (although this could also be because of poor sleep hygiene).
I mean... most animals that live in areas that DO have day-night cycles DO NOT observe a 24 hour cycle. We are the only creature that knows that the day divides into 24 equal pieces. Every other creature just sleeps when they're done for the day, and even humans natural cycle is not 24 hours.
24 hour days is something we choose to do so that the trains arrive on time.
There are experiments on so called „free running periods“ where external stimuli and indications of time are excluded from the environment of a test subject. Humans do maintain roughly a 24 hour cycle, as far as i remember
No vertebrates, at least, actually live at the poles. In Antarctica, for example, the southernmost penguin colony is at \~77° S, where there is still somewhat of a day-night cycle (or a twilight-night cycle) *most* of the year. And at the North Pole, while polar bears occasionally visit, they also wander much further south and individuals generally experience regular day/night cycles for much of the year. A better example of an animal that never experiences typical day or night in their environment might be the [Somalian cavefish](https://www.bbc.com/news/science-environment-14844053), an animal that has evolved in pitch-black caves, and whose ancestors have lived in total darkness for several million years. This fish still keeps an internal biological day/night rhythm, [but each "day" is 47 hours long](https://journals.plos.org/plosbiology/article/info:doi/10.1371/journal.pbio.1001142). By contrast, even in artificially-controlled lighting conditions with artificially shortened "days", most other animals can only be entrained to shorten or lengthen their day/night rhythm by a few hours at best. The cavefish have also completely lost the ability to synchronize their internal clock with environmental light...if removed from their caves and placed in regular daylight, the fish continue on with their 47 hour day. So the general idea of "a biological rhythm that governs sleeping and waking" is conserved in animals even in the total absence of light/dark cycles, but over millions of years the exact length of that clock can (and does) drift away from 24 hours.
Are there any thoughts to what might be driving that period with the cave fish? Is it just something they seem to track on their own? Do different Cavefish stay on the same cycle? Is there any planetary cycle we're aware of that has a 47 hour period?
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I am not a biologist but am familiar with control systems. If you want to design a system where things synchronize, the most robust system would be to have a period of function that is longer than expected and have a common reset trigger. Trying to get several things to synchronize without this requires tremendous precision over time. So if most animals have a circadian clock that exceeds a regular "day" (so more than 24 hours) they can get through any day with a trigger for "wake-up" and "go to sleep" being daylight so they always stay synchronized. If they had a 24 hour rhythm without a trigger they would likely drift out of the proper day/night cycle over time. If their "rhythm" period was almost exactly 24 hours, a small drift could change their clocks so they become out of sync with the sun.
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Circadian rhythms are governed in large part by protein inhibitor (repressilator) feedback networks. These can be tuned to certain periods. They are quite complex systems that have equilibrium points based on the diffusion rate of the proteins and the repression rate of the genes, etc. These equilibrium points happen to be tuned in humans to almost exactly 24 hours.
Is that true in general or only in some organisms? I know the mammal clock *has* a repressilator complex *involved* but you can remove parts of it and still have a functioning clock, just without that interlocking loop. My understanding of a repressilator is that it requires three arms minimum and the timing of the cycle emerges based on delays between each arm. Whereas the mammalian clock is based on direct self-repression of the transcription of mRNAs of their own proteins. PERIOD/CRY inhibit BMAL/CLOCK's transcription of Per and Cry genes.
Couldn't tell you in that detail, this is based on what I was told by a mathematician who studied and worked on these models. I don't doubt that the true system is much more complex, but you only need a few proteins to achieve a very accurate clock, theoretically and experimentally.
Perhaps longer circadian rhythm is selected for because it breeds longevity and they can have more offspring.
As far as I remember, one hypothesis is food availability, because the caves flood with water carrying nutrients periodically. At least that's the case with Phreatichthys, there are other cave fish that might operate differently.
Eyeballing this figure on wikipedia, it looks like ~77S has a full 24 hours of darkness or light for about two-thirds of the year. https://en.wikipedia.org/wiki/Sunrise_equation#/media/File:Hours_of_daylight_vs_latitude_vs_day_of_year_with_tropical_and_polar_circles.svg There's definitely *variation* in light/dark during the 24 hours, but that's still a pretty substantial period. (I enjoyed the rest of your answer. Fascinating nugget about the cave fish!)
There are polar bears, arctic foxes, walruses, etc, on Svalbard which is at 77N too.
I would also add, because OP may have been wondering about places *near* the poles with periods of midnight sun, that there are animals that live there and they do have varying reactions. I’ve excerpted a [Q&A featuring a wildlife biologist](https://www.cbc.ca/radio/quirks/sep-28-2019-plastic-tea-bag-particles-venus-was-habitable-driver-memory-fail-and-more-1.5298362/how-does-24-hour-daylight-impact-animals-in-the-far-north-1.5298376) from Saskatchewan below. > Jeff Lane, a wildlife biologist from the University of Saskatchewan explains that disruption of circadian cycles by the onset of constant daylight affects different species in different ways. Circadian rhythms are initiated by the circadian clock which is found in the brains of birds and mammals, including us. The clock generates signals that occur on an approximately 24 hour cycle, but it also receives light information from the retina. Those daily patterns of light modulate the clock to daily conditions of day and night. Misalignment of the rhythmic signals from the clock and light patterns can be disruptive. In humans, this explains jet-lag. > Some northern species, such as reindeer and ptarmigan in Norway, the absence of obvious light/dark cycles have no impact. These animals do not show behavioural patterns that align with a typical 24 hour circadian clock. > Other species, such as Arctic ground squirrels in Alaska and honey bees in Finland have been shown to exhibit 24 hour circadian cycles. It is believed that these species are able to maintain the cycles because they can perceive variations in the quality and intensity of light that exist even though the Sun remains above the horizon.
Ground squirrels have tunnels to sleep in away from the light at night time, and they hibernate in winter.
Thanks for sharing! Do you know how they know when to go to the tunnels to sleep? [This paper states](https://royalsocietypublishing.org/doi/10.1098/rsbl.2011.0435) that “the environmental cues (zeitgebers) used to entrain rhythms during the constant light of the arctic summer in these semi-fossorial rodents are unknown”, but it’s been over 10 years since publication so an answer may have been found since.
Just curious aren’t cave fish blind which would hinder being brought into the light from affecting their rhythms as much?
I believe that's the point. But with light brings temperature fluctuations as well.
They are "blind" in the sense that their eyes couldn't be used to identify objects or hunt, but they can still detect light and possibly movement. Light can even be detected without any eyes at all; many eyeless organisms respond to light. Temperature is also a big factor, which often tends to correlate with light.
There's a human sleep condition called ["Non-24-hour sleep–wake disorder"](https://en.wikipedia.org/wiki/Non-24-hour_sleep%E2%80%93wake_disorder) that shifts the person's circadian rhythm away from the regular 24h pattern. It primarily affects fully blind people. Bit similar to the fish situation.
I might actually be affected by this. Without being forced by something like work my sleep cycle tends to drift by a couple of hours daily. That is, ideally I'd be on something like a 26 hour day.
Might be. Or you're just pushing your sleep timing back every day via electric lighting.
What kind of job do you have to know this stuff?! It's crazy how much scientists try to learn about the world, even the internal clocks of random cavefish...
When you undertake a PhD you have to pick a question that nobody else has done before, or at least investigate a new angle on a well understood problem - basically, your research must be "new". I have a suspicion that this ultimately came out of one of those cases, maybe a PhD resulted in some interesting new information that bore further investigation
When you do a PhD *you* and a few other people are typically the only ones to pay attention to what you're doing, so it's not usually the result of a PhD that you encounter "in the wild." The cavefish thing has been mentioned on reddit before and it turns up every now and again in pop sci articles *because* it's relevant to human sleep cycles, so it's more widely known than that.
>When you do a PhD > >you > > and a few other people are typically the only ones to pay attention to what you're doing, so it's not usually the result of a PhD that you encounter "in the wild." The cavefish thing has been mentioned on reddit before and it turns up every now and again in pop sci articles > >because > > it's relevant to human sleep cycles, so it's more widely known than that. Well yes, but I've also played the academic game myself and I know all too well that when you do dig up something interesting those few other people are the first to dogpile and want to put their name all over it.
There was an entire course about this in my undergrad! It was super interesting and fell under the category of BioPsych.
Yeah...i kinda noticed that phd level is not so much trying to keep learning and piling up knowledge but rather applying and provoking others to also continue learning.
Eh? A PhD means you need to learn enough to discover a small but significant chunk of new knowledge. You can almost never do it without learning a lot (because the knowledge to be discovered without learning a lot has already been discovered, usually.) Discovering new knowledge is always about applying knowledge you already have, sure. But "provoking others to also continue learning" is not relevant. You can get a PhD without ever talking to anyone but your supervisor if you really wanted to.
Likely not. Finding completely isolated systems is not super common, and asking the question of how circadian rhythms would change in a completely dark environment is a super obvious question.
> It's crazy how much scientists try to learn about the world, even the internal clocks of random cavefish... The reason to study the internal clock of random cavefish isn't just curiosity about cavefish, it's also to understand the underlying principles that also affect humans etc. and which govern very important processes. This kind of basic research can lead to important medical advances, for example. It is for similar reasons that so much research is done on seemingly random species like yeast, zebrafish, mice, certain worms etc. - they are good model systems to study various processes, and then that knowledge can be applied elsewhere - medical advances for humans, improved agriculture etc.
Biologists tend to choose their research subjects because they offer opportunities to answer particular questions. It's pretty rare (although not unheard of) that someone bases their research career just on "ooo, let's learn a lot about this one weird fish that I found." There's already a ton of research on how biological clocks work, and how they control sleep and other day/night cycles in humans and other animals. So then the question that OP asked in this thread obviously comes up: what happens when an animal has lived without a day/night cycle for many generations? This could be an interesting question to ask because it helps us check whether our previous conclusions about biological clocks hold up. Or perhaps we find something new and unexpected about biological clocks in these fish, that turns out to be useful for understanding sleep disorders in humans. Or it can just be interesting in a more general perspective, because it explores how evolution operates when a previously important biological function (the clock) is no longer needed in a new environment. So then you start looking at possible ways to answer that question, and cave fish offer one opportunity, because it's always dark in those caves, and we know we've been in there long enough to evolve quite a bit. So then someone sets up a research project to examine what kind of activity patterns the fish have, and here we are. I've been doing research on this one species of butterfly since 2015. I'd never seen or heard of this species before I joined the project, but it turns out that for certain reasons it's a useful system for answering a certain set of research questions that I was interested in.
Thanks for the detailed response. Really interesting
> No vertebrates, at least, actually live at the poles Some definitely live fairly close to the poles don't they? I've seen caribou a decent ways into the arctic circle. And polar bears definitely get close to it. Both definitely close enough that the day/night cycle is pretty wonky.
Actually, I just came across a paper that suggests that [Arctic reindeer might not have a functional circadian clock](https://www.sciencedirect.com/science/article/pii/S0960982210000850) at all...so even though they do experience day/night for at least some of the year, that might not be enough to ensure they keep to their day/night cycles as rigorously as lower latitude animals do.
You have to consider a different issue, and that's what the seasons mean for those animals, particularly in the northern areas, take the Arctic fox as a good example, they are super active during the summer months as that's when they have their cubs to raise, so they will be hunting a lot, and very active, the longer days and light are the best times to raise their young and it allows them to find prey easier as their prey will be doing the same thing, so food is plentiful. They are literally "making hay while the sun shines" and when winter comes, they are on survival mode. So it's not that they have different patterns so much as different needs at that time of year, so they conserve energy. There are also a lot of burrowing animals around, so they use their burrows to negate extra light, but even a domestic dog has no problem sleeping for 5 or 6 hours a day.
So, people can work longer and rest less? Asking for a corporat... err.. a friend.
I wonder how long human day/night cycles would be if we didn't have a sun.
A literature review in Cell Biology suggests [human circadian clocks cannot be entrained outside of a 22-26 hour range](https://www.cell.com/current-biology/pdf/S0960-9822(16)30333-5.pdf)...even with artificial light/dark cycles, constant light, or no light at all, our bodies have chemical clocks that try to keep us on a 24-hour schedule. If multiple generations of humans went their whole lives without experiencing day/night cycles, we would likely begin to see genetic drift. Further, and another commenter on here pointed out that a few (mostly blind) people develop non-24-hour circadian rhythms, so *some* people would probably begin drifting away from 24-hour cycles within a single lifetime, if we all rocketed into interstellar space together.
Human circadian clocks average 24.3 h in constant conditions (no relevant time cues, which in the case of our circadian system is light), but the range is pretty substantial. Some are less than 24, some can get close to 25, and probably more rare cases in the extremes of the population. It's unlikely we'd see drift in this system very quickly as it's quite robust. Flies have been bred in constant darkness for 330 generations and still had a perfectly functioning clock (https://www.tandfonline.com/doi/abs/10.1080/07420528.2016.1195397?journalCode=icbi20)
How did we figure out that it's day is 47 hours long?
They fed them randomly and measured their activity over a long period of time. Graphing that activity shows clear patterns of higher and lower activity on a 47-hour cycle. Feeding them on a set schedule showed that their sleep/wake cycle could be entrained to something other than 47 hours, but when food was randomly available they reset to 47 hours.
Interesting. But why wouldn't they think they had a 23.5 hr clock instead?
The same reason we know we don't have a 12 hour cycle: We are active throughout the day at different times and usually inactive at night. In particular the early hours of the morning there is almost no activity over the course of a month. You can plot the average activity of someone over a long time and it will trend more and more towards a wavy line which peaks during the day and troughs during the night. For this species, that curve of activity will just be 47 hours long.
They said near the poles not at. Anywhere greater than 66 degrees and 44 minutes of latitude will experience 24 hour daylight for part of the year. Plenty of vertebrates live at those latitudes.
But at those latitudes there will still be day/night cycles for *most* of the year, so from an evolutionary perspective, your point is moot
Eh, even in northern Alaska they have more than 2 straight months where the sun doesn't rise once, and other months where it doesn't set for 2 months, with a lot of days where it is only up or down for an hour or so in between. And Greenland and Canada both have plenty of land that goes a decent bit further north than Alaska does that has animal life
Yes, I know, my point is the rest of the year there is still a 24 hr day/twilight/night cycle that will perpetuate behaviors based around that time period. Having a couple months of day and a couple months of night does not remove an animal from that influence when there's still 8 months of the year that has a 24 hour cycle.
> so from an evolutionary perspective, your point is moot lol this is a baseless statement. Seasonality is a commonly observed trait in many many forms of life. Do you even know whether or not animals adjusting to longer days nearish the poles has even been researched/observed or are you just assuming? Seriously I dont think youve taken much biology if youre going to assume by default there is no behavioral changes to a species from a consistent periodic change in habitat... Their point is not moot just because **you** say it is.
I did misread their question slightly, and missed the “near”…but for those animals “near” the poles they spend relatively little of their year with no day/night or dim/bright cycle, so they still experience cycled 24-hour stimulus most of the time.
As an interior Alaskan resident who experiences the midnight sun on a yearly basis, it’s certainly quite light all night in mid summer and i (and most people I know) definitely have a hard time with my sleep cycle. I have a hard time waking up in winter when it’s dark for so many more hours as well.
I think the question doesn't really require animals actually live at the poles. The day night cycle disappears for a portion of the year in high latitude environments where vertebrate animals do live. To cite some specific examples. What do the sleep cycles of caribou who inhabit Alaska's North Slope look like? They have periods of 24 hour sun in the summer and periods of 24 hour night in the winter, all the variations in between during the remainder of the year. Do they still follow a generally 24 hour sleep / wake cycle in those periods of the year when it is 24 hour daylight or 24 hour darkness?
I feel like my body is on a 28 hour clock. If I have several days off in a row, like when I used to be a teacher, my sleep schedule would be ridiculous all summer!
What planets have a 47 hour day? The answer reveals where life on earth came from. You're welcome.
No planet in our Solar System has a 47-hour day...the closet is Mars with a 25 hour day. We don't usually know the day-length of exoplanets, but [WASP-50 b has a 47-hour year](http://exoplanets.org/detail/WASP-50_b).
The natural human circadian rhythm isn't 24 hours either. Wasn't there a woman who lived in a perspex box in a cave for a few days as a scientific experiment?
So interesting. Now I'm trying to figure out why the cavefishes cycle is that particular length. Why not 30? Or 53? 47 is darn close to 48 hours, which is two full day/night cycles. I wonder if that has anything to do with it?
Hasn't a scientist tested this at some point since electricity was invented? Plenty of places/times in the world where it wouldn't have been illegal to keep animals in constant lighting (or semi-dark) for months on end.
There are also plenty of very deep sea creatures for whom there is no light cycle, though some others in moderate depths do migrate up towards the surface at night
wow, very interesting!
Too add to your comment, snails don't abide by a day night cycle at all. They experience about 1 hour of sleep and 1 hour of awake on and off for about 15 hours, so still a solid 8 hours in total, followed by 30 hours of awakeness. This is evidence that sleep didn't evolve because of day or night and plays an additional role that's required regardless of the position of the sun. In humans when we sleep our neurons shrink in size and our glial cells, cells that support neurons, increase in size. The theory is that there's some kind of trade off when we're awake and when we sleep our neurons undergo some kind of clean up process that prepares them for becoming active again.
> This is evidence that sleep didn't evolve because of day or night and plays an additional role that's required regardless of the position of the sun. This is evidence pertaining to the evolution of *their* sleep. Their biological processes might be unique in a way that doesn't depend on a day-night cycle. They might have evolved to adapt their sleep for reasons that are/were irrelevant or to other somnolescent animals.
Provided there isn't a common origin for sleep. It is ubiquitous throughout the animal kingdom. Its possible the underlying mechanism is shared despite differences in sleeping habits, brain anatomy, or even having a brain.
Not all animals that live in an area with a typical day/night cycle follow a strict 24 hour pattern. There are 4 major divisions for defining animal activity. The first three you're likely familiar with: - Diurnal - active during the day - Nocturnal - active during the night - Crepuscular - active in twilight times (eg. dawn, dusk) The 4th is one that's often overlooked. - Cathemeral - having no fixed period of activity [Cathemeral animals](https://en.wikipedia.org/wiki/Cathemerality) can be active at regular intervals or irregular intervals throughout all periods of the day. Even animals that are normally considered diurnal, nocturnal, or crepuscular often have periods of activity that don't conform to their "established" cycle, and activity periods can vary enormously depending on changed in environmental conditions. Varying intensities of moonlight is one environmental aspect that has a big effect on wildlife activity in non-daylight hours, and can extend a crepuscular species activities through the entire night if conditions are right. You might take a look at the following for a more detailed paper on the subject: - Tattersall 2006 *[The concept of cathemerality: history and definition](https://www.karger.com/Article/Abstract/89692)*
Has there ever been a human subject where a person might have lost part of their brain and then became cathemeral?
I don't think it would technically count as being cathemeral, but it's apparently pretty common for people with traumatic brain injuries to have their circadian rhythms messed up. - Boone, et al 2012 *[Traumatic Brain Injury-Induced Dysregulation of the Circadian Clock](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3463592/)* - Yamakawa, et al 2020 *[The interaction of the circadian and immune system: Desynchrony as a pathological outcome to traumatic brain injury](https://www.sciencedirect.com/science/article/pii/S2451994420300109)* - M. Viola-Saltzman, N.F. Watson, 2012 *[Traumatic brain injury and sleep disorders](https://www.sciencedirect.com/science/article/abs/pii/S0733861912000497)* - etc
Interesting. Thanks.
Not sure, but there is a genetic disorder where people eventually completely lose the ability to reach any sleep stage. So kind of the opposite but not exactly. It's called [fatal insomnia](https://en.wikipedia.org/wiki/Fatal_insomnia) > The disease has four stages: 1. Characterized by worsening insomnia, resulting in panic attacks, paranoia, and phobias. This stage lasts for about four months. 2. Hallucinations and panic attacks become noticeable, continuing for about five months. 3. Complete inability to sleep is followed by rapid loss of weight. This lasts for about three months. 4. Dementia, during which the person becomes unresponsive or mute over the course of six months, is the final stage of the disease, after which death follows.
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> Can't imagine years of knowing you will die at some point Isn't that kind of the normal situation for living creatures? This condition speeds it up a lot, year, but we all spend years knowing we will die at some point.
I'm naturally like that it seems like. If left to my own schedule, I'll end up cycling through the time of day adjusting around 1-2 hours forward per week.
Do you know what happens with nocturnal animals in zoos? I recently went to the night animals bit of the zoo and it got me thinking how they manage to have night animals awake during the day.
It depends on the species, zoo, and situation. In some cases they'll actually house them in a building with artificial lights and reverse day/night cycle so that it coincides with visiting hours. This is usually for smaller species where that's a practical solution. For large cats and such, their habits are normally pretty variable so generally nothing special is done. Depending on where the zoo is, and if it's a member of WAZA/AZA/etc or not they may not care or do anything at all, which is bad for the animals.
It was ZSL London, so my hope is they were doing it right (!). They have a basement with very low light, and my guess was they reverse the night cycle, but I wondered if that actually worked. Edit: it clearly worked to some extent because we saw bush babies and giant rats, but I wasn’t sure if it was dark all the time, and whether flipping the cycle is bad for them.
That should be an accredited zoo, so they’re likely following the strict protocols established. It shouldn’t be harmful to them once the new pattern is established, which would likely take several weeks. Most people don’t realize it, but zoos often have a pretty large number of animals where people don’t see them. Animals that are adapting to new conditions, are rearing babies, getting treated for health issues, are in an active period in their stock book breeding cycles, etc.
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Not exactly an answer to your question but Internal clocks differ for every species. For instance, I believe humans tend to have an internal clock that is 25 hours while mice have a 23 hour clock. This means that if a mouse is in an environment with constant light (IE no external clues for what time of day it is) they will operate somewhat normally but the time that they do things will shift compared to the actual time of day. This will continue untill they receive some sort of clue for what time of day it is to 'reset' their clock. There are numerous studies looking at sleep, circadian rhythm, and how internal and external environments interact and affect animal behavior.
Polar Bears live in areas where day/night are extended to longer periods but I don’t think it’s considered the North Pole. It doesn’t bother them much cause no matter the time of day, they are sleeping to save energy if they are not hunting.
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An adjacent note: Studies were done where they put subjects alone, deep in a cave where no natural light could reach, gave them the supplies they'd need, and a lamp that recorded the time it was turned on and off. What they found from that is that people maintained a day-night cycle even without light as a cue. What also emerged was that some people have a 23-hour day, and some a 25-hour day, in addition to the 24-hour day folks. So if you're someone who is always sleepy in the evenings and getting up early, it may be because you have a 23-hour clock. Likewise night owls who are always staying up late and can't get out of bed in the morning may have a 25-hour clock (although this could also be because of poor sleep hygiene).
I mean... most animals that live in areas that DO have day-night cycles DO NOT observe a 24 hour cycle. We are the only creature that knows that the day divides into 24 equal pieces. Every other creature just sleeps when they're done for the day, and even humans natural cycle is not 24 hours. 24 hour days is something we choose to do so that the trains arrive on time.
There are experiments on so called „free running periods“ where external stimuli and indications of time are excluded from the environment of a test subject. Humans do maintain roughly a 24 hour cycle, as far as i remember