From an evolutionary perspective, how did our species evolve to have derp sleep for long hours? I understand that sleeping conserves energy, but it would also make humans vulnerable to predators and not getting killed should take priority over saving energy
In: Biology
Have you gotten killed during your sleep?
Had your ancestors?
Sleep has numerous restorative benefits and it does not make one as vulnerable as you’d think (otherwise, all our ancestors would have been killed).
Especially not if you congregate and sleep near others, as humans tend to do.
All animals do. It seems to be essential for the healthy functioning of our brains to have inactive time – time that some studies have shown is necessary for neurons to be able to form new memories and proper function.
It seems to be common to all animals, though some animals have evolved an ability to send one half of their brains to sleep at a time
[https://en.wikipedia.org/wiki/Unihemispheric_slow-wave_sleep](https://en.wikipedia.org/wiki/Unihemispheric_slow-wave_sleep)
Chances are IMO we did not evolve that need because we have evolved intelligence and communicative language to teach others e.g. to stand watch over the tribe on the lookout for danger. That sort of thing.
In addition to the biphasic sleep pattern and morning dove vs. night owl explanations, keep in mind we need 7 hours of deeper sleep while other great apes sleep for 9-10 hours a day of lighter sleep.
So we did evolve with that constraint in mind, partly by reducing the total hours we were asleep during the day.
The communal nature of humans meant we could get away with sleeping longer.
Some more recent discoveries suggest that your circadian rhythm is actually partially embedded in your genes, and the primary theory to why is from our early days; some people in a community would go to sleep and wake up earlier while others later. This meant that a community could always have at least a couple people awake at any given hour to alert the rest should a threat come about.
Additionally, early humans; even running into the early modern era; didn’t actually sleep the full 8 hours that we do today, instead having what was called “first sleep” and “second sleep.” Essentially, they would sleep for about 4 hours, wake up for a few hours, then sleep for another 4 hours. Among other things, this biphasic pattern would help to ensure that rule of “someone should be awake at all times.” Biphasic sleep was likey only abandoned during the Industrial Revolution, as artificial lighting and scheduled shift-work led to humans either having to go to bed later or getting up earlier, squeezing the two 4 hour blocks of sleep into the one 8 hour block we have today. What’s weird about it is, despite it disappearing just 200 years ago, we had completely forgotten about it until very recently, and even now there’s some loose interpretations as to how it actually worked.
> Vulnerable to predators
Yes and no. Humans live in group. That makes it extremely dangerous to attack them, even for the most fearsome animals, and even while they are sleeping. On top of that, they hold a grudge. If an animal kills one human, the humans are going to take revenge on their whole family.
As a result, almost all land animals learned that it’s not a good idea to attack humans. Humans don’t really have any significant predator.
Regarding your main question, it’s not super clear why we need sleep, but it’s clearly not unique to humans. All animals with a brain that is bigger than a peanut sleep. And although we know the consequences of a lack of sleep, it’s still not very well understood why those happen.
It’s also worth nothing that chimps, gorillas, or even cats sleep more than we do. So it’s not like humans are an outlier either.
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