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
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.
I could nitpick some of the details there, but instead maybe I’ll just ask what point you’re trying to make? A healthy human can still pick out something small way better than a goat.
A healthy human can still pick out something small way better than a goat
Sometimes…
Depending on if you’re talking about motion, color, or lowlight.
But since when is “optimized” just “better than a goat”?
Like, cats easily blow everything else out of the water.
Optimised just means designed for something at the expense of other parameters. We lost our tepetum lucidum at some point in evolution, probably for the 3x-ish resolution gain, while becoming much more shit in lowlight in the process. That’s a tradeoff, but a good one for a tree-based diurnal frugivore.
Cats (for example) still have theirs, which means light as two chances to hit their retina, but means there’s an upper limit on how clear an image can be, exactly because there’s light bouncing around. It sounds like 20/100 is typical for them, from a quick search. Cats are traditionally thought to be dichromats, as well.
Cats are traditionally thought to be dichromats, as well.
And humans usually have three, but sometimes it’s two, and even rarely 4…
With that much variations (including other ways) it’s hard to say human eyes are optimized for any condition.
There are very few examples of things in nature that are truly optimized for all of its environment.
Humans are just too widespread with too much variation to say we’re optimized for anything.
We just have too much in species variation.
We actually have less genetic variation than most animals. There was a lot of bottlenecking in the paleolithic. And what little we do have is still mostly confined to Africa, because the rest or the world shared common ancestry as we left our original continent.
Like, 1 in 200 people is colourblind, or something? I don’t think that’s a reasonable argument that we’re not trichromats.
You’re talking genetic variation, I’m talking phenotype variation…
Like, 1 in 200 people is colourblind, or something?
…
Again, you’re talking genetics, where it is clearly broken down in 2,3,4.
However like pretty much everything else, it’s not that clear cut just because the plans were.
Two people with the same amount of different types of cones are not guaranteed to have the same rod/cone ratio. Even when they have similar genetics for the ratio, things rarely go according to plan as a human develops.
Like, you know that’s why facial symmetry is attractive right? It shows that things on both halves went according to plan. Which especially for women is a huge bonus for reproductive health.
Especially for something made up of a whole bunch of small things like rods/cones, it’s not even perfect for identical twins.
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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.
Hmm. Are we talking a high canopy, and fairly level ground? I feel like I’d definitely break an ankle if I tried sprinting otherwise.
I never had too much trouble, but sometimes things hiding in tall grass would surprise me, and in heavily treed patches I’d occasionally hit a low branch I didn’t notice.
I also have to account for the fact that there was some light pollution, and I could always see skyglow from towns in the distance. I doubt land ever gets close, prehistoric or not, but in the darkest conditions that happen at sea apparently you can’t see your own hands.
I feel like I’d definitely break an ankle if I tried sprinting otherwise
Yeah, we played paintball even, but stopped because one guy ran straight off like a 6 foot mini cliff. A couple of us were chasing him and he just disappeared. Was freaky as shit like that scene from LotRs.
I also have to account for the fact that there was some light pollution
Yeah, I’m talking really hillbilly stuff, zero light pollution.
but in the darkest conditions that happen at sea apparently you can’t see your own hands.
A ship gives off a lot of light pollution, but even without that, between the water reflecting and nothing blocking light, it’s brighter out there unless there’s heavy clouds cover. And even then it’s gotta be a lot of clouds and rough waves or else the light would still be refracting some.
Now a watertight compartment on a ship with the light switch on the outside?
Yeah, that’s complete darkness. It’s not just “can’t see your hand in front of your face”. It’s the absolute and complete absence of light. That’s total darkness.
And it fucks with you very quickly.
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
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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
Oh trust me, I know way more than you think. It is literally my job to study circadian rhythms. I can very comfortably say that you’re wrong
The intensity and the wavelength of light influence entrainment.[2] Dim light can affect entrainment relative to darkness.[15] Brighter light is more effective than dim light.[12] In humans, a lower intensity short wavelength (blue/violet) light appears to be equally effective as a higher intensity of white light.[11]
https://en.m.wikipedia.org/wiki/Light_effects_on_circadian_rhythm
For anyone else, I won’t try to change your mind.
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.
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.
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.