Regarding sleep quality, why did humans evolve to require full darkness?
Regarding sleep quality, why did humans evolve to require full darkness?
I know evolution is governed by chance and it is random but does it make sense to "ruin" sleep if there's light? I mean normally, outside, you never have pure darkness, there are the moon and stars even at night. In certain zones of the Earth we also have long periods of no sunshine and long periods of only sunshine.
I don't know if my question is clear enough but I hope so.
Bonus question: are animals subject to the same contribution of light or lack of it to the quality of sleep?
A question that I'm an expert in!
I study circadian rhythms (the process that is responsible for getting us to sleep in the night). Specifically, how circadian rhythms influence how easily we catch diseases, but that part is less relevant to the question.
So since Earth rotates and has day/night cycles, life on Earth evolved to try to predict when the day and night comes. That's what circadian rhythms do. This is really important, since day and night aren't just associated with lightness/darkness. Day and night are associated with a ton of different environmental differences. For instance, it's colder at night, so animals need a way of keeping warm at night. There's more UV light at day, so animals need a way of resisting DNA damage in the day. There's some evidence that the bacteria in the air are different at day vs. at night, so animals will need to have different levels of immune system alertness.
We as humans live in artificial houses with artificial lighting, so we can lose track of why this is really important. But if you've ever went camping or tried to stay out at night you'll probably understand why it's really important for animals to be able to predict the time.
Circadian rhythms end up getting reinforced on a community level, since if it's easier to see in the day, an animal is more likely to forage in the day. Then predators will notice that prey is more plentiful in the day, so it will also be more likely for predators to hunt in the day as well.
Anyways, the end result of all of this is that animals have a huge evolutionary pressure to pick either the day or night to be their active period, which is the time where they look for food and in general just be awake. And whatever they don't pick, that's their rest period, the time where they sleep and recover.
But how do animals know that their circadian rhythms are predicting the correct time? Imagine a mouse in its burrow - it wouldn't be able to tell what time it is just by looking at the sky. And even just stepping out for a second to check would be very dangerous if it ended up being the wrong time. Animals need a way of reading what time it is when their out and about and then correcting their circadian rhythms if the rhythm is inaccurate. There's a lot of different measurements that animals use to read the time, but the key here is that the measurements that they pick must change significantly between day and night. In other words, it must be a very obvious signal, like "oh, I see this signal, so there is no doubt that the time is day."
Vast majority of the time, the most obvious signal ends up behind light. And it makes sense - if you see bright light, that is the clearest indication that it is day outside. So for many animals, light is the primary measure that animals use to read the time.
So to wrap back around to your question, it's not necessarily that light ruins sleep because evolution just decided to go "nae nae," it's because predicting time is incredibly important for keeping animals and humans alive, and up until very recently, light has simply been the easiest and best proxy for the time
And to answer your bonus question, yes, other animals have their sleep messed up by light too
Due to being a "night owl" myself, I guess I've always been a little doubtful regarding circadian rhythms myself; but your explanation did a great job of boiling it all down to the most significant component parts. Thank you! I really liked your summary!
No need to use quotation marks - it is scientifically confirmed that night owls and early birds exist (among a number of other, less-well-known circadian types). We call them chronotypes, and it's an active field of study. Unfortunately it's not something that I specialize in, so I can't comment too much on it.
However, it is very well acknowledged in the field that modern society is built on an early bird schedule and that completely screws over night owls. (To my memory, night owls tend to score lower on tests, pursue higher education less than early birds, tend to be less promoted and generally less successful than early birds. Inversely, night owls tend to do better in evening classes than early birds.)
animals have a huge evolutionary pressure to pick either the day or night to be their active period
Cats: I reject your reality and substitute my own. I'm not sure if there are any other animals that are crepuscular, but I assume there are.
Very neat write-up; thank you!
Yeah, crepuscular animals are weird. They have circadian rhythms (the circadian clock is incredibly well conserved across vertebrates and to a lesser extent, across invertebrates), but I'm not actually entirely sure how their circadian clock work to get them to wake up at dawn/dusk
Sorry to piggyback onto your comment, and I know you can't give medical advice, but I wonder if you have any insight into a problem I have with sleep.
I'm early 40s now. One of my earliest memories, aged around 4, is not being able to fall asleep. I've tried EVERYTHING over the years. Sleeping pills are a guarantee if things are getting squirrelly, but give me severe rebound insomnia the next day. When I do fall asleep, it's like I can sleep for way longer than is normal (so either cause of sleep debt or poor sleep quality).
I've always joked that maybe I should be on a planet with a 28 hour day. But I also know that my lack of normal sleep is potentially storing up huge problems like increasing my risk of cancer, heart disease etc.
Melatonin kind of helps. But no matter what I do... My sleep pattern goes out of synch.
I've gone through school, 'normal' 9 to 5 jobs, relationships, all a big struggle as I have to perform at a normal level despite not having slept for 24+ hours fairly regularly.
I can do everything 'right' (no light in the evening, exercise, healthy diet, no excitement in the evening, no caffeine, mild sleep supplements) and still find myself unable to sleep. What the frick is wrong with me... Am I doomed to continue like this? I just want to sleep like a normal human being!
You're right, I can't give medical advice. But having abnormally long or short circadian days is a known thing - called circadian diseases. It's not really my specialty, so I can't comment too much on it, but my understanding is that many of them are genetic. These genetic variations can cause the circadian clock to run slower or faster than normal (which happens to be adjacent to what I study, so I can talk about it in excruciating detail if desired)
The Familial Advanced Sleep Phase Syndrome (FASP) is one such genetic circadian disease that gets a lot of attention among the circadian field, but you almost certainly don't have it, since FASP makes your clock run shorter than 24 hours, whereas you seem to imply that yours runs longer.
The key thing to remember is that the circadian clock is not psychological. There is an actual, physical, molecular clock running in your brain and in nearly all the cells in your body. If this clock has imperfections, then that will directly lead to consequences in your circadian rhythms and your sleep cycle. The circadian clock is a real thing that people with the right equipment can measure and read. It wouldn't even be particularly hard - just a blood sample or a swab would be sufficient. To be honest, I myself would like to study your cells to see if there really is anything out of place, but that would probably break so many research and ethics rules.
Anyways, to answer your question, I would recommend getting a medical opinion - it might be worth specifically bringing up that you suspect you have a circadian disease. I'm not too sure about treatment options, since my impression has generally been that we kind of don't have any treatments for circadian diseases. But it's not really my specialty, so maybe there is. My memory is that melatonin is a masking cue, which basically means that it makes you sleep but it doesn't actually affect your circadian clock (which probably explains your poor experience with melatonin).
I have the exact same thing.
Ever heard of
https://en.m.wikipedia.org/wiki/Delayed_sleep_phase_disorder
?
We had a community on Reddit which I kinda miss, really small there as well but I'm not going back for it.
Does this mean humans in far north climates have different methods of determinating sleep times? Because I'm originally from close to the equator and I'm the summer I'll be awake until near midnight when at least we get some dusk, but the nightless days really screw me up
Ah, so this goes more into the nuance of what exactly determines the time of the circadian clock. It is very well documented that animals in the arctic circle still have circadian clocks even if it's perpetual light or dark. I left out for simplicity that the level of light matters - that is to say, if there's a time where it's slightly dimmer and a time that's slightly brighter, that is enough to adjust the circadian clock to the correct time. The adjustment process will be slower and weaker than usual, but it does happen.
Also, I hinted that animals do take in multiple measurements to determine the correct time, and that plays a role in this case. In general, light tends to be the measurement that animals will default to, but where light variation doesn't exist, animals can and do utilize other measurements to determine the time. Eating (among other things) turns out to be a relatively strong signal, so circadian rhythms end up being somewhat self-reinforcing. After all, I would expect that you only eat when you're awake.
But in general, circadian rhythms and the ways that animals adjust their rhythms to the correct time is a huge rabbithole
I've severe sleep problems.
I live on a quite a northern latitude. Finland, but the very southern end of it. (The Arctic circle only starts about at the most northern 1/3 of Finland)
I'll upload two pictures, taken from the same spot at different times.
Which one is later, which is earlier? One is taken at 00.30 and one at 2.30. No peeking at the exif data before guessing.
Sunset or sunrise?
Couldn't tell you, as we don't really have those in the way you do.
There's some evidence that the bacteria in the air are different at day vs. at night
This is really interesting, do you have more info on this to share?
Yeah, sure! This happens to be my field of research.
So I was referring to this particular paper, which unfortunately (to my knowledge) didn't get much follow-up.
Tangentially, there is much other evidence that circadian rhythms have evolved in part to deal with differences in microbial pathogens at the day vs. at night. However, whether it's because the composition of bacteria in the atmosphere is different, or because animals are more likely to get themselves exposed to pathogens when they're foraging, or a mix of both, is unclear. My favorite paper that demonstrates this effect is this one, where the circadian clock affects how strongly the immune system responds to bacteria in the lungs. I'll also include the seminal paper here that first kickstarted the idea that immunology is fundamentally circadian, although frankly I didn't like how the paper was written. It looked at how mice responded to Salmonella infection at the day vs. at night and found a difference in immune response that then led to a difference in how severe the infection got.
Really cool, thank you!
We didn't...
"Full darkness" isn't even a real thing in nature. It's hard to tell with light pollution, but even in the absolute middle of nowhere with no artificial lights, you're going to be able to see fairly well. Even with no moon, starlight isn't just an expression. And on a full moon it can be surprisingly "bright" if you're just out there for a while.
It's not like climbing into a cupboard, shutting the door, and sealing all the cracks with duct tape.
You may be used to needi g full darkness to sleep, but that's a learned habit. I guarantee if there was nothing you could do, it wouldn't take you long to adapt your "requirement" of total darkness.
but even in the absolute middle of nowhere with no artificial lights, you’re going to be able to see fairly well.
I'm not sure I'd say fairly well. Maybe always well enough to not walk directly into a tree in otherwise open terrain. A full moon will be comfortable to walk around in, but new moons happen just as often, and sometimes the moon is below the horizon.
Source: Have walked around in the country at night.
I mean, my night vision was always better than most...
But growing up as kids we'd be sprinting thru the woods playing tag at like 10pm summer nights, not a single electric light in sight
You're not going to recognize someone 100 yards away, but you're not walking around with your hands in front of your face to make sure you don't run into anything.
If you're under an open sky, or even a primitive shelter, you're not in complete darkness.
same, and i agree with you.
The experience of people working the night shift, who use blackout curtains to sleep during the day, would disagree.
But that's for a relatively highly regimented sleep cycle. If you slept and worked completely at your leisure, you might end up with one shorter sleep period at night, and one even shorter nap during the day. And without any day-night cycle at all, some people naturally adopt cycles of varying lengths.
The experience of people working the night shift, who use blackout curtains to sleep during the day, would disagree.
Wow, I didn't know my own experience disagreed with me...
Or that during my childhood when my dad was swing shift, he was apparently a freak of nature too...
But that’s for a relatively highly regimented sleep cycle. If you slept and worked completely at your leisure, you might end up with one shorter sleep period at night, and one even shorter nap during the day. And without any day-night cycle at all, some people naturally adopt cycles of varying lengths.
Again, human variation is a big thing.
But an individual will change their sleep schedule as they age, which is another supporting point for what I'm saying.
Evolutionary biologists hypothesis that it was so out of an entire tribe of early hominds, at least some members were likely to be awake. It wasn't an inate guard duty rotation. But kids and middle age went to bed early, teens went to bed super late, and by then the elderly were waking up.
If something happened, someone screamed and everyone woke up. And the fires stayed lit all night.
I'm not talking about myself, melatonine, is synthesized by the body when it's dark, light can reduce or stop the synthesis.
Nope.
It's a very specific wavelength of light that inhibits it.
https://en.m.wikipedia.org/wiki/Melatonin#Regulation
That's why there's "blue light filters" on electronics these days. That wavelength isnt included with moonlight/starlight.. maybe on a big full moon there's be some.
And why people prefer soft yellowish lights when relaxing and not the bright ass LEDs.
This is untrue - we have explicitly evolved to sleep in the dark. Sleeping in the light is a learned behavior that's more or less an exploitation of a loophole in the circadian clock
Are you saying that sleeping under full moon levels of illumination is not something animals would have dealt with since time immemorial?
...
A specific wavelength may effect you..
That wavelength is not present in moonlight/starlight, which is not "full darkness".
For the vast majority of human evolution, "full darkness" wasn't safe, and wasn't even really possible.
I understand what you and OP are trying to say. And you both kind of have the general idea but none of the details.
Like how you got taught basic things in 6th grade, but by 12 grade you're learning what you thought was the whole truth, was just a general overview.
Which wouldn't be bad if you recognized it, but loads of people want to insist the short summary the learned as a child is as deep as it gets
Full darkness is most certainly a thing and is more of a thing then light..light is artificial. Remove the sun...what do you get, full darkness. Light is added, darkness isn't.
Same with heat..everything is cold unless heat is added.
Cold and full dark are forever, heat and light are techcially temporary.
“Full darkness” isn’t even a real thing in nature.
And
It’s not like climbing into a cupboard, shutting the door, and sealing all the cracks with duct tape.
So I thought it was pretty clear I meant that to get "full darkness" where you really can't see, requires extra steps to intentionally make it happen. Just that for the vast majority of human evolution, we weren't really capable of it, and would have no reason to even try.
Keep in mind modern "nighttime" is very light polluted, so it's likely a lot darker most of the time, than you would think.
Also I think it's less about darkness, and more about the transition from light to dark.
Also yes the poles are weird, keep in mind that the poles are basically inhabitable, and northern Alaska is barely habitable.
OTOH far away from light pollution you get so used to the dark you clearly see the galaxy in the middle of night.
So I think it was rare to have perfect thick cloud coverage so not often a pitch black dark.
i mean yeah, but even then starlight is basically fuck all. The moon overpowers those, it just makes the sky look pretty. That's it.
Cloud cover would be primarily lit by moonlit. And even then, moon light is very dim. Just look at early moon light towers used to light up residential areas early in the electrification period.
It's literally the difference between being in your home, at night, and you can't see shit. Vs you can just barely make out where things are, and navigate properly.
Also semantic point, light pollution is not "dark" that's why you can't see any stars. Ever looked at a highway lit with LEDs recently? They have tons of light pollution that can be seen as what's referred to as "sky glow" My point here being, when you go outside in a light polluted area at night, it's literally not dark.
We're diurnal, and have eyes optimised to see maximum colour and detail instead of well in dim light (at least by mammal standards). It makes sense we'd gravitate to fairly dark conditions to sleep, because while nature at night is not perfectly unlit, it's still pretty dark. Darker than a developed-world urban area will ever get, for example.
That being said, many people are completely capable of sleeping in a bright area, myself included.
As for the bonus question, yes, the hormones at least work backwards in nocturnal animals. Melatonin wakes something like a shrew up.
We’re diurnal, and have eyes optimised to see maximum colour and detail instead of well in dim light (at least by mammal standards)
Human variation.
There's two main structures in our eyes.
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Rods: take large amounts of any wave length of light
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Cones: take in a very small amount of a specific wavelength and only that wavelength
Most of the area (like 95%) are rods. And there's a couple (usually three) types of cones.
Some people have more different types of cones, and can see more differences in color. Some have less types meaning less cones overall even.
But the eye won't just have more blank spots. So it fills in with more rods.
This is actually related to why the further away from the equator people got, the lighter their eyes got.
With longer variation in day/night cycle, it was advantages to let as much light in as possible. That outweighed the downside of too much light during the day, as that could be solved with hat brims, or that age old move where you make a visor with your palm.
By limiting the amount of light going to your rods, your cones get less "washed out" and that's how we get more detail/colors.
But even in a single population, there's going to be a lot of human variation. Rod/cone distribution has a high amount of variability even when genetics are steady. Genetics has a large effect, but it's not like the body always follows directions closely.
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Uh...we didn't evolve that way. We evolved to have eyelids, which make it dark enough whenever.
But ... we don't require full darkness?