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?
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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.
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.
Lol, yup, that sounds right. I did that once, although it was only like 3 or 4 feet, and I didn't like it one bit. Is was a sinkhole or something too, because it was cliff all around, and I had to find a spot to climb out. I didn't visit that area again.
I forget where I heard about the sailing thing now. That would be a 1 on the Bortle dark sky scale, though.
i found i did indeed need to have hands out because i can’t see much at all in deep country at night on a new moon. maybe i just don’t have great night vision.
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
I'm here to learn, I admit I'm ignorant and that's why I love asking questions here. Maybe it's me but your comment came across a bit rude.
Anyway thanks for engaging here and providing answers and sources.
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.
Yes, but your wikipedia link doesn't prove that animals are only sensitive to blue light, only that they are more sensitive to blue light. That is a very well-documented phenomenon. But there is plenty of evidence that red light can entrain circadian rhythms as well, dating well back to the 80's. There has even been a study that identified different mechanisms of entrainment to low-wavelength and high-wavelength light in bacteria, which you can find below. My point is that it is very scientifically irresponsible, and in fact, blatantly wrong, to claim that humans are sensitive only to particular wavelengths of light, when in fact humans and other animals are sensitive to all wavelengths of visible light.
Beyond that, I don't necessarily know why you seem to be claiming that the intensity of the ambient light does not matter for photic entrainment, when this is a highly documented and, in fact, highly studied phenomenon in the circadian field. Yes, the moon reflects light, but this is dim enough that mammalian SCN's can interpret the difference between that and the full daylight. See below for some papers that look into light intensity and their effects on entrainment.
Here is some reading if you are interested:
- https://www.science.org/doi/10.1126/sciadv.adh1308
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4311741/
- https://pubmed.ncbi.nlm.nih.gov/7435660/
- https://www.sciencedirect.com/science/article/abs/pii/S0269749119358233
- https://www.brainanddevelopment.com/article/S0387-7604(00)00146-7/abstract
- https://pubmed.ncbi.nlm.nih.gov/9428526/
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.