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Regarding sleep quality, why did humans evolve to require full darkness? (lemmy.ml)
submitted 4 months ago by linucs@lemmy.ml to c/askscience@lemmy.world
23 comments fedilink hide all child comments
 

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?

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[–] CanadaPlus@lemmy.sdf.org 0 points 4 months ago* (last edited 4 months ago) (1 children)

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.

[–] givesomefucks@lemmy.world -1 points 4 months ago* (last edited 4 months ago) (1 children)

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.

  1. Rods: take large amounts of any wave length of light

  2. 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.

[–] CanadaPlus@lemmy.sdf.org 0 points 4 months ago (1 children)

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.

[–] givesomefucks@lemmy.world -1 points 4 months ago (1 children)

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.

[–] CanadaPlus@lemmy.sdf.org 0 points 4 months ago (1 children)

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.

[–] givesomefucks@lemmy.world -1 points 4 months ago (1 children)

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.

[–] CanadaPlus@lemmy.sdf.org 0 points 4 months ago (1 children)

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.

[–] givesomefucks@lemmy.world -1 points 4 months ago

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.

[–] givesomefucks@lemmy.world -1 points 4 months ago* (last edited 4 months ago) (5 children)

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.

[–] Fiivemacs@lemmy.ca 0 points 4 months ago* (last edited 4 months ago) (1 children)

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.

[–] givesomefucks@lemmy.world -1 points 4 months ago

“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.

[–] linucs@lemmy.ml 0 points 4 months ago (1 children)

I'm not talking about myself, melatonine, is synthesized by the body when it's dark, light can reduce or stop the synthesis.

[–] givesomefucks@lemmy.world -1 points 4 months ago

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.

[–] CanadaPlus@lemmy.sdf.org 0 points 4 months ago (1 children)

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.

[–] givesomefucks@lemmy.world -1 points 4 months ago (1 children)

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.

[–] CanadaPlus@lemmy.sdf.org 0 points 4 months ago* (last edited 4 months ago) (1 children)

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.

[–] givesomefucks@lemmy.world -1 points 4 months ago

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.

[–] catloaf@lemm.ee 0 points 4 months ago (1 children)

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.

[–] givesomefucks@lemmy.world -1 points 4 months ago

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.

[–] Contramuffin@lemmy.world 0 points 4 months ago (1 children)

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

[–] givesomefucks@lemmy.world -1 points 4 months ago* (last edited 4 months ago) (1 children)

...

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

[–] Contramuffin@lemmy.world 0 points 4 months ago (1 children)

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

[–] givesomefucks@lemmy.world -1 points 4 months ago

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.