system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly determines how much sleep different animals need?

i.e. Humans need 8 hours but other mammals will sleep much more or much less.  Does it have a direct correlation to the brain size of the animal vs body weight?  Or neural density?  Diet?
gpt
We actually don't really know. Even within species there's huge variation.

But without going into anything too much:

Your question is a loaded and not an entirely accurate premise.

Prior to WWII, 8 hours of sleep wasn't a standard.

Plenty of countries have had "siesta cultures" where it's normal to take a 2-3 hour nap during the day.

But even in those where daytime naps were uncommon, bimodal sleep seemed to be the standard for most humans:

You go to sleep in the late afternoon or around sunset, wake up in the middle of the night and chill out for a couple of hours, then go back to sleep until sunrise.

Some scientists believe this window is when humans were most likely to procreate, helps strengthen familial bonds without the pressure of a day's work to be done and allows the completion of the small number of tasks or hunts that can be done better at night


Even beyond that, many humans have almost reversed circadian  rhythms and melatonin production with night time promoting wakefulness and responding to blue light in the way most humans do to it's absence - that is to say "night owls" and "early birds" are an actual biological phenomenon.

It's only recently that we've enforced this rigid "8 hours sleep, 16 hours awake" routine and it's often quoted as the reason so many people feel tired throughout the day, struggle to fall asleep or wake up throughout the night - we're going against our programming.

The point being that even our species has a variable and unusual sleep requirement and we're not exactly sure why - it's difficult to predict why there's variance between species when there's so much within species.