Submitted by ebb5 t3_y7s95g in askscience
The day is 24 hours and we typically are awake for 16 of them and sleep for 8. If humans lived on a planet where the day was 48 hours long, would we still be awake 16, sleep 8, wake for 16, sleep for 8? Or would we be awake for 32 and sleep for 16? Did our bodies adapt to the length of the day?
Here is a link to an article about the study you mentioned. The abstract is fairly easy to understand.
I just read a little of this. I find it fascinating that 2 subjects ended up on a 32hr cycle with alternating 8hr and 16hr sleeps and 24ish hours awake and active. Now this might sound too crazy, in fact, sounds like a fun weekend, but they thought they were awake and sleeping their normal amount.
Not too surprising to me, since I have a strong suspicion that I'd end up just like them if I was in a similar study.
When not actively trying to maintain a schedule, I'll tend to sleep for about 12 hours and be awake for about 20 hours. Makes it a pain in the ass to keep a normal schedule without constant alarms and reminders.
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Not too surprising to me, since I have a strong suspicion that I'd end up just like them if I was in a similar study.
When not actively trying to maintain a schedule, I'll tend to sleep for about 12 hours and be awake for about 20 hours. Makes it a pain in the ass to keep a normal schedule without constant alarms and reminders.
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Our Human circadian system (and most vertebrates and many inverts, though often through different biological mechanisms) is completely governed by light availability. Light presence degrades a protein that is a part of our circadian “clock”, and sets a rhythm that tells us to sleep. You can set a circadian rhythm in humans with just an hour of light exposure at the beginning and end of the day time. Interestingly, hibernation and different types of torpor (prolonged moments of rest) are totally unlinked to the circadian system, which solely governs our sleep based on the position of sun in the sky/daylight. When you think about it, that makes sense - they often go into burrows, where sunlight reaches very little. Hibernation is actually more so linked to cold; though whether that’s a direct link to cold or a result of decreased nutrient availability varies between species. It been shown that some species at the very least actually are able to complete shut down their circadian system from functioning during hibernation to prevent interference with the two systems.
A really interesting part of animal life in the arctic circle though is that not all species hibernate. Arctic reindeer, which don’t hibernate, have been observed to have a much less controlling circadian system compared to closely related, non arctic species. They’ve essentially shut off their circadian system as an adaptation to maintain essential biological needs throughout the day. They are able to graze periodically every few hours throughout the day as their body needs, even without the presence of sunlight. They do still have higher prevalence of melatonin during the night, which indicates that they can still denote when the seasons change, but it’s not linked to a distinct sleep period, unlike other ruminates. Unfortunately, that is an adaptation to the environment that would take generations and generations to arise in a species randomly, and as humans are relatively new to the arctic environment in an evolutionary sense, that adaptation has not been observed yet in native human populations of the arctic. Humans are still beholden to the circadian system in the arctic, meaning that in periods of full dark there is more pressure to sleep in a free running period of 25 hours.
Also, because is was mentioned, the 25 hour free running period you spoke of (free running means the natural period of sleep observed in no-light conditions) is something that has been observed in many species. It’s not actually odd like you said, because when you arrive at the mechanic behind it the answer makes sense. The link is that it’s only present in diurnal (active during the day) species. Nocturnal species tend to have a 23 hour free running period. Because the protein that signals sleep degrades in light, and that degradation takes time, this allows diurnal and nocturnal animals to be awake at the right time when entrained to the sunlight, sometime around daybreak and sometime after sunset respectively. Theoretically in some exoplanet that might have a 20 hour day, species that evolved there with the same circadian rhythms would have a 21 and 19 hour free running period because of that.
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Wikipedia is a little light on the exact source, but they make the claim that the 25 hour study was flawed, in that it allowed artificial light. If I'm to believe their references, a better estimate for human circadian rhythm is 24h11m. And it varies by gender.
I mean the alternative is having the subjects spend days in complete darkness.
Its weird but foreign people mocking us spanish for taking naps made me cut them out entirely. Such irony that siesta is slowly getting common place along cultures due to climate change. This past summer was so hot that countries that weren't used to such a heat wave started to adopt mid day naps. My point is that climate pays a huge factor aswell.
>Its weird but foreign people mocking us spanish for taking naps
IDK about the rest of the world, but we Americans have been brainwashed into thinking that working to much and sleeping too little is some kind of virtue. I remember hearing about Mediterranean cultures having a big lunch as their main meal and then having a nap because it's the hottest part of the day, then go finish up the day's work when the sun isn't beating down so hard. I thought it made a lot of sense and I wish it was a thing in the southern US.
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I'm american and you can have my naps when you pry them from my warm, sleepy hands.
Also do not forget that the older you get, the less your body naturally produce melatonin. Which is one the reasons old people can't sleep as much. It does not necessarely mean that they don't need it. I would even suggest the hypothesis that it create a negative feedback loop that accelerate the aging process as you get older.
I, for one, wish that I lived on a planet with 27-30 hour long days. I'm a "night person", and every night is a struggle to get to sleep, and every morning is a struggle to wake up. If left to my own devices (ie. if I didn't have a job) I would stay up an hour or 2 later every day and sleep 2 or 3 hours later every morning.
For a few months when I was 18 or 19 I was utterly ... free. For lack of a better word. No job, no responsibilities. I slept and woke when and for how long I wanted. Was staying in a friend's basement that was pitch black 24/7.
I found that I naturally drifted towards something like 28 up, 12 down. It varied a lot, but was way longer than the normal 16/8 shtick we're stuck with. Also, I never felt more alert and alive before or since.
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This just prompted an interesting question in my head, as a recent mother, I was told to make sure to get the baby accustomed to night vs day to help them “develop their circadian rhythm”. So…. What if this study was conducted on humans straight from birth (terrible idea and doubt it would ever be moral to do it); however, would the results be drastically different if a baby never experienced night/day from the beginning?
We have a cluster of cells called the suprachiasmatic nucleus that serves as the internal clock. It’s hard to say whether it’s based on the length of the day because it’s evolved over time with nothing more than a 24hour day. I think OPs question is asking the mechanism of the SCN and it would respond to different period lengths, which has yet to be discovered. If the fundamentals of the internal clock are true, it wouldn’t matter how long the day is because our internal clock is telling time no matter what.
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The study misunderstood the results. We have an internal clock. It is about one day long. Then it gets reset. If it doesn't get rest it gets chaotic. It doesn't find a different day, it doesn't find a regularity at all.
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That’s super interesting. I’ve always felt like my body was on a 25 hour clock… and every couple days I have to do my own little mini daylight savings time to try to reset back to 24
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In the old days, before electricity people would wake up in the middle of the night and mingle before going back to their second sleep. Can read more about it here: https://www.bbc.com/future/article/20220107-the-lost-medieval-habit-of-biphasic-sleep
Later on the changed the sleeping patterns changed during the industrial revolution.
I was unemployed for like 2 months and I naturally started waking up around 2 and just like doing whatever then going back to sleep an hour or two later and that’s the only time in my life I’ve felt well rested and wasn’t tired all day
I did the same while unemployed for 3-1/2 months. Seemed like a wacked-out sleep schedule, but it was effective. Now I'm back to the 8-5 grind and it's take a week to change.
SAME!!!!!!!!!! DURING summer breaks when i was in uni that was my default sleeping pattern. absolutely amazing
What time did you go to sleep before you woke up at 2?
I did this in college and I went to sleep at 9, woke up for an hour around 1, and then went back to sleep at 2 and woke up around 6 or 7.
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What if you felt more well rested because you work working?
Same. The real issue is that if I know I have work in the morning I stress about not getting enough sleep and can’t get back to sleep the second time and end up with 3-4 hours
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Bimodal sleep may be limited to Europe:
>Historical evidence suggests that “until the close of the early modern era, Western Europeans experienced two major intervals of sleep bridged by up to an hour or more of quiet wakefulness” [33 ] (see also [30 ]). Our results suggest that the bimodal sleep pattern that may have existed in Western Europe is not present in traditional equatorial groups today and, by extension, was probably not present before humans migrated into Western Europe. Rather, this pattern may have been a consequence of longer winter nights in higher latitudes. In this view, the “recent” disappearance of bimodal sleep was not a pathological development caused by restricted sleep duration, but rather a return to a pattern still seen today in the groups we studied, enabled by the electric lights and temperature control that restored aspects of natural conditions in the tropical latitudes.
Most weekends I sleep from around 5pm to 10pm then I’m up for a few hours and I go back to bed about 1am and then get up around 5-6am
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The bbc article suggests otherwise:
> Biphasic sleep was not unique to England, either – it was widely practised throughout the preindustrial world. In France, the initial sleep was the "premier somme"; in Italy, it was "primo sonno". In fact, Eckirch found evidence of the habit in locations as distant as Africa, South and Southeast Asia, Australia, South America and the Middle East.
So it’s unclear, at least, but the bbc article also suggests compelling evidence that biphasic sleep exists even today in Africa, and other places without artificial light
> Back in 2015, together with collaborators from a number of other universities, Samson recruited local volunteers from the remote community of Manadena in northeastern Madagascar for a study. The location is a large village that backs on to a national park – and there is no infrastructure for electricity, so nights are almost as dark as they would have been for millennia.
> The participants, who were mostly farmers, were asked to wear an "actimeter" – a sophisticated activity-sensing device that can be used to track sleep cycles – for 10 days, to track their sleep patterns.
> "What we found was that [in those without artificial light], there was a period of activity right after midnight until about 01:00-01:30 in the morning," says Samson, "and then it would drop back to sleep and to inactivity until they woke up at 06:00, usually coinciding with the rising of the Sun."
> As it turns out, biphasic sleep never vanished entirely – it lives on in pockets of the world today.
This is really interesting! Thank you for the link.
This only started to change 100 years ago around 1920s.
Ever heard of the sleep ambassador Dr. Matthew Walker
Legit
I used to do similar back when I went out to late night clubs. May be asleep till midnight or 1 then hit the doors an hour or two later, come in around sunup sleep till 10-11 am ( or maybe noon or 1). It's like an even split 24. I also used to have to go to 3-4 clubs in a night to collect for bands I booked, when I knew no use in being there till closing time. Did that for years.
So my body naturally does that. I wake up about 4 hours into the night, get up and do a few things, and then go back to sleep.
I thought I had some like weird form of insomnia. Nope! Just a regular biphasic sleep schedule.
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Wow this was such a great read!! I never had any idea about this at all. Tysm for sharing!
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Natural selection tends to promote whatever is successful. Either our body adapted around successful sleep/wake cycles or our ancestors figured out the best sleep/wake cycles to respond to their bodies' demands and environmental factors (it's harder to hunt and gather food during the night without a readily available light source).
Most predators were more active in the nights than the days too, so it was safer to go out in groups during the days.
if a modern human now would go to live on a different planet they would maintain their current sleep schedule
if humans were to live on a different planet for millions of years our biology would probably set itsself to the day night cycle of that planet
altho i once heard something about humans having a 25h inner clock but dont pin me down on that
I remember reading that at some point nuclear submarine crews were on an 18 hour day, and only the radio officer and captain were on 24 hour days. I had the opportunity to ask a crewman a year or so back and was told that was no longer the practice. But it sounds like it was.
I also once read that the people controlling Mars rovers lived on a 27 hour day. That may have stopped with the newer more autonomous rovers.
What do you mean by "18 hour day"? When the caption would hit hour 19 of day one, woukd that mean the crewmen would be on the first hour of day two? If so, what was the point?
Don't think about counting how many days have passed, more like its an 18 hour routine... instead of a 24 hour routine...instead of sleep for 8 hours then awake for 16, they would be asleep for 8 hours then be awake for 10, then start over. So from an outside perspective your wake-up time rotates.
I think the point was the keep sailors better rested... instead of potentially being awake for 16 hours and a bit groggy, the longest you should be awake (barring sleep disturbances) would be 10 hours. If the whole ship is rotating at this rate, its the same number of crewmen covering the same number of work shifts, but less chances of a sleepy mistake.... in theory
Wouldn't that run into increased shift handoff errors?
Possibly. But theoretically the crew rotating out is a bit better rested, enabling them to transfer their duties for effectively.
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Humans inner clock is not exactly 24h (sometimes more sometimes less depending on the person) but it's influenced by external factors such as light, time of eating etc., so you always lock to 24h (it's also how your body can adapt to changing timezones).
If you leave someone long enough in a place without natural light or external points of reference, they will go back to their "inner" clock which is based on the actual fluctuation of circadian proteins and hormones
> more sometimes less depending on the person) but it’s influenced by external factors such as light, time of eating etc., so you always lock to 24h
Just wanted to point out that a small percentage of people do not lock to 24h
Non24 sleep wake disorder is most commonly found in the blind, but some sighted people also suffer with it.
Some biological cycles are easier to up regulate or down regulate so can be more adaptive for the baseline to err slightly in the direction that is easier to correct. Just let the outside stimulus do its magic to keep things sensible.
Million years? Absolutely no. Sleep schedule is something you can change, fix, break on demand with some effort and heavily depends on external factors.
For starters sleep cycle is not something hard-coded to the human system. Closer thing to that would be sleep ratio. For example 8(sleep) -16 (awake) is the most common but it's not even close to be the same for everyone. Many have 7-17 many 9-15. Many are unable to keep it in a 24h cycle, they will either sleep every day earlier or later and every now and then they will need to fix it (the latter is actually pretty common issue for many people nowadays.
Going back in the past as many people already mentioned is that humans before society required proper schedules were using biphasic or even triphasic sleep schedules. I still belive (personal experience) that biphasic sleep is superior however it's very problematic to follow in a modern society due to being often required to stay awake for 15+ h
Long story short like I said there are a ton of examples and each individual will adjust differently but I'm very confident that if for example we move to a planet with 36h cycle almost no one would keep their 8-16 cycle. It doesn't divide the day perfectly (and each day being the same) and it will be extremely wonky. I'm sure most would go to biphasic sleep cycle and do something like 6-12 and 6-12. Or maybe even triphasic 4-8 three times.
Some might try to have monophasic with naps or other crazy scenarios but doubt many would try to keep earths cycle
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yeah i know, the moon is getting pushed outwards, therefore needing longer for one circle arround earth which causes friction between then which makes earth spin slower
There are other sources but there was this study done on circadian rhythm. It was conducted in a cave and the findings pointed to humans (or at least the one in this experiment) having closer to a 48 hour day when devoid of clocks ot sunlight. source
Kind of sort of. It was a misleading study and didn’t fully understand the mechanisms behind sleep. They were devoid of sunlight, but not light altogether. They still used artificial light, which disrupted the degradation of proteins in the circadian clock which normally would signal you to sleep during periods of darkness. They essentially deprived their bodies of sleep by extending the period where the signal to stay up (light) was present. While a 48 hour period can be entrained, biological assays have shown that there is still a biologically maintained free running period of around 25 hours throughout the body. This is present in almost all diurnal organisms (organisms that are active during the day). Nocturnal organisms tend to have a free running period closer to 23 hours. Both of these have been observed in environments completely devoid of all light, and also in blind humans with complete enucleation (lack of eyeballs), who are unable to have any photo reception. The 48 hour period works because it is a natural multiple of 24, and allows some synchronicity with biological functions throughout the body that were determined by the 25 hour free running circadian clock. While we can function under those conditions, and will entrain to that period, by no means is our biological determined sleep cycle specifically 48 hours.
For sure, this isn't my favorite study mostly due to the sample size. I do think however, this may be one of the most relevant studies to answer or at least offer some understanding to OP's question. Personally I find anything having to do with societal conditioning vs. what our bodies actually need super fascinating.
Is there more information on this? I've always had issues with sleep and have determined my rhythm is between 27 and 36 with the solstice and equinox having the largest sway of insomniac and especially heavy sleeping.
I'm tempted to relate it to my severe MDD but with no income it's impossible to see a doctor of any variety it seems, so all self-research and testing. I'm leaning this way as for instance I can end up having more than 36hrs a cycle but never less than 24 due to being awake especially long with either especially long sleep along with not being able to return to sleep ever unless I'm especially exhausted.
This study was done twice, and both came to approximately the same results.
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So, what you’re talking about specifically is the principle of entrainment. Organisms that use light to determine the length of sleep have a chemically based circadian clock where a protein degrades with light exposure and causes the “clock” to send out a wake signal, and vice versa for dark. These organisms can entrain to any length of day, given a certain period of light/dark cycle. You can shorten the sleep cycle by exposing organisms to a shorter day/night cycle, and lengthen it via a longer day/night cycle. Humans can adapt to a 48 hour day, in fact we have done so artificially as some have already mentioned, but I think it should be made clear that the human biological clock as it stands does not change as a result of that, it still naturally has an approximately 24 hour period (23-25 hours depending on sleep behavior across species). 48 hours actually works pretty well for us because biotic functions that work in a 24 hour clock can still synchronize fairly well with that multiple of 24. Getting away from that multiple can push some biological functions with 24 hour periods out of synch with the natural rhythm, but we will still have sleep cycles that match the light cycle because of entrainment.
Now, if we had originated and or evolved on a planet with a 48 hour day, that would be a different story, though I think that’s obvious enough. Theoretically if humans from earth lived on this planet for generations we would eventually evolve a natural clock that aligned with the 48 hour period of the sun.
Where are these proteins located? In the eye?
In the eye there are photo receptors that respond to light, I think that much is obvious, but more specifically there are specific types of photoreceptive cells called retinal ganglion cells (RGCs) which contain melanopsin, a pigment that is energized by a certain wavelength of light. That energizing causes the the RGC to send a neural pulse, called an action potential to a region of the brain called the suprachiasmatic nucleus (SCN), located near the hypothalamus. Now, interestingly while RGCs do play a role in sight, this specific pathway to the SCN is different than the pathway to the optical cortex(which is at the back of the head, not near the hypothalamus), and is why some blind people can still respond to light and maintain a circadian rhythm without being able to actually see or process what they are seeing.
Now, this is complex and it’s been a minute since I studied the specific pathway, so I’ll explain it incredibly simply. Once the action potential reaches the SCN, it innervates cells there, which is where the proteins I mentioned are created. Those proteins act as transcription factors, which cause the transcription of many genes essential for sleep, such as melatonin. And I think that’s where I’ll leave it, as I’ll get it too convoluted, and I’ve probably already said some things wrong. Just know that’s generally how the system functions, and that it’s the SCN that acts as the hub which releases hormonal signals to synchronize the entire body’s cells individual clocks.
If you’re interested in the subject, there are much more in depth articles and videos on the circadian clock I’d recommend looking into.
That's super interesting thank you for the response. Glad you mentioned blind people because that was gonna be my next question. I googled it and it turns out blind people without any light sensitivity do suffer from messed up circadian rhythms. Wild how complex and specific the processes of our bodies are. Always blows my mind when I learn about a new one.
I suggest reading a book by an actual sleep scientist instead of relying on reddit armchair specialists.
I recommend "why we sleep" by Mathew Walker. He breaks down a lot of the research on sleep, a lot of it was surprising in that, even in the last decade they have learned a lot more about what is going on in the brain during sleep than I expected. I assumed it was still mostly a mystery.
But if I had to give an answer to your specific question I would guess if the new planets rotation was close enough, we would adapt to it but 48 hours would be too long. There are waste products that build up in your brain just from the activities of sleeping, and when you sleep cerebral spinal fluid is pumped through your brain to flush them out. Glial cells even shrink a bit to facilitate this process. That's happening in deep sleep in other stages memories are being shuffled from short term to long term memory, in rem there is a lot of dreaming, they speculate its purpose is emotional stability, There's a lot going on but ask yourself how you feel after being awake for 48 hours.
I remember reading an article when the micro channels of csf brain-cleaning function was discovered...neat stuff.
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Have you ever heard of the experiment where the people were underground in complete darkness (light was on only when they were awake) and their sleeping schedule/internal clock changed to like a 48 hour cycle where they were awake for 36 hours and asleep for 12 or something? Thought that was cool, and if that’s possible then other changes are probably possible right?
We're biologically wired to, more or less, follow the pattern of the sun. Light in general, especially blue light, suggests to our brain we should be awake, and darkness suggests sleep. But idk if we could stay up 48 hours and then sleep for 15, I don't think our bodies are built to function like that. We might eventually adjust to follow the new pattern, but it certainly wouldn't be any time soon after arriving on the planet
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Although a bunch of studies found 25 hours or even much longer (median 25 hours), with a lot of variability from person to person, another later study suggested that early ones found so much variability due to residual light contamination and that the length was closer to 24.2 hours
Ensuring complete darkness for the duration of the study, they also found less variability from person to person.
There are also studies showing how much light is needed to change your circadian rhythm to a shorter or longer or different (think jet lag) cycle.
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Side note - by modern standards planetary colonization and terraforming are silly sci-fi ideas born from a time of the space race. It's akin to contemporaries of Jules Verne describing the future where everyone takes a personal biplane to work at their floating blimp office.
The more likely scenario based on modern or near future tech (which may be as silly as above examples by the time we actually get to that point) would be building artificial habitats in space. A megastructure 3/4 the size of our moon could house roughly a trillion humans. These habitats are imagined to have an artificially maintained 24 hour day/night cycle.
Or just… better designed buildings that house more people efficiently?
We will never live in space. Gravity is expensive to fight and space tries to kill everything. Anyone considering space colonization suffers delusions of grandeur, probably due to the lead, mercury, and microplastics they ingest, as we poison ourselves on earth.
Humans as all warm blooded creatures generate heat and the planet has a limited capacity to reject that heat - thermodynamics dictate that the Earth has a limited carrying capacity for humans. This is putting aside all the pollution that you mentioned that we create, which will likely kill us way before we cook the planet with our farts and sweat.
Gravity doesn't need to be fought. Spin up a cylinder of 500m in diameter at 2 rpm and you have earth gravity. Luckily, in space that cylinder will keep spinning.
For radiation shielding - 5m layer of dirt (formed from asteroid mining slag) solves that issue.
Saying we will never live in space is like saying we will never: circumnavigate the globe, fly or go into space.
Sadly, living in space is easier to imagine than everyone throwing their plastic bottles in the trash and not on the ground.
Isn't all the heat human bodies generate derived from the sun? If humans weren't here, the heat that gets stored as energy in sugar, muscle, fat etc would instead just turn to heat immediately. We aren't creating heat from nothing.
Correct, however.
Energy in = energy out + energy stored
When the Sun is creating sugar the Earth can reject the remaining heat. But when we start converting that sugar into heat, the Sun does not turn off, it keeps supplying the same amount of heat as before. So now if we take the constant supply of heat from the Sun and we add the conversion of stored sugar energy into heat, without increasing the Earths capacity to reject heat, at a certain rate of sugar burning we will overwhelm the Earth's heat rejection capacity and the internal temperature of the system will rise.
But we're planting plants and raising livestock. As humans expand we're the ones adding more stored energy. The net energy is still the same. Every bit of sugar burned was heat saved earlier in the growing season.
But as we burn that sugar, the Sun keeps supplying the same amount of heat as it was while the sugar was being created.
Imagine a river, a damb and a reservoir.
The river flows to the reservoir and through the damb. All of the water came from the river, but if we release too much from the reservoir at once, the damb will overflow.
But the sugar absorbed the energy when it was grown
It's net zero. We can't be generating more heat than the sun is supplying, that breaks the conservation of energy. It's like the fallacy in The Matrix that humans could generate energy via body heat. It's always zero sum.
If I grow corn in June, the corn is absorbing the heat from the sun and converting to sugar. The net effect of growing corn reduces the total heat of the Earth.
When someone eats the corn and their body temp is increases the heat is released. Since all across the earth we are pretty much constantly growing and eating this mostly all evens out. And even if it's not perfectly even, due to seasons, you still over the course of the year have net zero heat vs if humans didn't exist at all.
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I don't know, but what I do know is that I tend to personally be awake for 20 hours or so, then sleep for 10 or so. This seems to be my schedule as of the past few years now, it doesn't matter if it's day or night. Thankfully, I don't have any jobs that require me signing in or on a schedule. (Web developer & UberEats driver).
I understand in the time before industrialization humans did a 2 sleep cycle. Go to bed at dusk and sleep for 4 hours or so and then get up and do some reading, hanky panky, and family talk time. Doctors of the time said sex between sleeps was the best for conceiving. When street lights were invented and industrialization came along, the church, government, and business got together and created the 8 hours of work, 8 hours to yourself, and 8 hours of sleep. Humans were never meant to sleep the way we are forced to today.
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I was reading (somewhere - I don’t remember where), that most people do better on a slightly longer than 24-hour day. They conducted a study of people while keeping them in dark caves. I know when I have no scheduled obligations, I end up flipping my hours around and sleeping a lot less.
In complete darkness people stayed up for thirty six hours and then slept twelve to fourteen hours.
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The people suggesting that we have a 48 hour sleep cycle aren’t completely right. The 48 hour cycle works because they were still exposed to artificial light when they wanted to be active. 48 being a multiple of 24, there was still overlap with biological functions and it’s an easy rhythm to entrain. Without light altogether diurnal organisms naturally fall into closer to a 25 hour cycle.
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there was a study with different sleeping patterns observed and it found that for the most awake hours (and no residual brain issues), you should have 20minutes of REM every 4 hours. Meaning you will be awake 22 hours!
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A few people have mentioned the study that showed a 25 hour day.
If you are setting up a control system, one way to do that is to set the hard reset at longer than expected and then have a soft reset trigger at the "best" time.
This means if the soft reset is missed for any reason, the second hard reset will kick in but not before the potential "optimum".
The sun rising in this case is the soft reset and if you miss that you get at most an extra hour so you are not too far out of rhythm.
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Mammals have internal circadian rhythms. Humans will drift up about 24.5 hours on average without sun, with significant variance.
We are amazing at syncing our clocks do another system, though I don't know how we'd handle 48 hours.
Separately, you assume we've always were 16/8. South America, mostly Mexico, used to an extended rest in the day time. Before the industrial revolution, many cultures would sleep from dusk till midnight, get up for a few hours and do community activities, and then sleep until early morning.
Modern sleep pattern of an 8 hour chunk at night didn't materialize until around the industrial revolution when time management and efficiency became more of an issue. 8 hours at night is not what humans naturally took to. For most of human history we had split sleep - or biphasic sleep - commonly referred to as First Sleep and Second Sleep, with a break in between where we caught up on reading or had sex. You can read accounts of first sleep and second sleep all throughout history from classical Greece and Rome to about the 19th century.
Humans have an internal clock of around 24.2 hours and varies from something like 24-25 (not normally distributed...the longer day tail is much longer). Sunlight is the primary driver of it through our SCN , and since sunlight is on a 24 hour cycle, we adjust to that.
Where exactly we set our sleep time vs sunset varies on how close our internal clock is to 24 hours with longer clocks being night people and shorter being morning people.
There have been studies on how far humans can adapt. The nicest I'm aware of are from Richard Kronauer's team and tested 23.5 and 24.6 hour days with different light exposures:
https://pubmed.ncbi.nlm.nih.gov/17684566/
We can maybe adjust to day lengths outside of that. I was a researcher on studies to determine what submarine schedules were viable for the Navy, and 18 hour days were outside of the range of what humans could adapt to.
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There was a study done to answer this exact question - I read it several years ago though so I can't remember what it was called. I believe it took place in conjunction with testing people in enclosed environments, such as those you'd see on a ship headed to Mars. They were kept in a space where they were deliberately were unable to tell night from day (plus no clocks, calendars, etc). I remember it being found that people tended to default to wake/sleep cycles longer than 24 hours, but I don't remember how much longer. Two days? I'm having difficulty finding it, but obviously NASA is extremely interested in this question as well.
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Assuming our species evolved with that new extended period of time, we would probably just sleep for like a few hours more, not too substantially much since when I’m exhausted I can almost easily sleep for more than 10 hours
As others have pointed out, we have an intrinsic circadian cycle that runs just slightly longer than 24 hours. Other organisms, such as mice, can have slightly shorter cycles at around 23.5 hrs. Hence the term circadian: from Latin circa, meaning "about" and dies, meaning "day".
This circadian rhythm is built-in and governs multiple functions, of which sleep is one. Temperature and cortisol secretion, for example, also oscillate with a similar daily period. We know this from some crazy time-isolation experiments where subjects lived in caves or isolated rooms with no indicators of time. Because our natural cycle is longer than 24 hours, these subjects slept a little bit later everyday and after some time they ended up drifting their sleep onset so that it occured during our daytime (they had no idea whether it's day or night). Later, it was discovered that exposure to light can reset this cycle so that it prevents this natural daily advance in sleep onset. This is why we follow a 24 hour cycle in the outside world where sunlight resets this cycle so that it begins at a new phase ever day.
Importantly, the maximum amount of time that this cycle can be reset is 3 hours per day. This is why it takes a few days for jet lag to really clear up.
On other planets, we would still follow our 25 hr circadian cycle but we'd reset it everyday depending on availability of light. If the day on that other planet was something be like 36 hours, my guess is that we'd be constantly jetlagged if we planned our activities around our circadian cycle.
On a related note, these time-isolation experiments found that the subjects began to do something weird after about a month where they would stay up way past their bedtime and then sleep for 15 hours without even realizing it. They then bounced back to a normal 25 hour cycle before getting desynchronized again and starting a crazy cycle 40 to 50 hours long. The researchers figured out that this was linked to body temperature at time of sleep onset. Basically, long sleeps always began at high body temperature and normal sleeps at low body temperature.
To be fair you should be sleeping for 8 hours every 24H period, not exactly 8 hours straight. More recent science is showing that sleeping 8 hours straight isn't as beneficial as sets of rest periods, 2 sets of 4 hours. We'd probably evolve differently if our timescale was 48 hours, if the ratio of sun and moon were the same, but we'd probably still have the same ratio of rest. 1/3rd of the day.
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theUturn2Yz t1_iswqedt wrote
I hope someone can chime in if they know of any species of animals that historically hibernate during winter and become active during the summer in the Artic Circle where the sun does not set for weeks. Maybe an expert can also talk about nomadic cultures in a similar context.
A quick tangent: there is a psych study where participants willingly were placed in an artificial residence (possibly in a building or underground) and left inside for months without the ability to tell what time of day it was from external stimuli. Basically, humans (and I would assume almost all animals) have an internal circadian clock that regulates bodily functions. For some odd reason, ours tends to run around 25 hours per day (not 24).
Also, there are examples of cultures predominately taking naps or socializing at various times of day contrasted to other cultures e.g. Spain and Turkey. Old people start to sleep at odd hours and sometimes require less sleep too.