system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes us fall asleep and why can't we decide when to fall asleep?

I mean we just lay there for 5 to 30 minutes and out of nowhere we just fall asleep. Why can't I decide when to fall asleep?
gpt
There's the physiological "why can't we", and then the evolutionary "why can't we" part to answer.
The physiological answer is, as already noted, that we gradually lose control of our consciousness and our muscles by hormones that we secrete when the time to sleep arrives.  The hormonal secretion also in turn depends on our body's state of excitation and mental state.  So it all kind of has to wind itself down in a recursive manner.

As for the evolutionary reason, that's a really good question.  Given how detrimental sleep deprivation can be to health, you'd think we'd have evolved the ability to just force ourselves to sleep over our worries, overthinking, loud noise, lights whatever.  For many of us insomniacs it would help if we could just push a mental button and send ourselves out to dreamland.
It might be that during most of our evolution, it was more important for your survival to *not* sleep if you had concerns and worries (about hungry animals in the dark, for example..., or enemies attacking).  Maybe this was more important for survival than getting a good night's sleep, so nature made it so that you couldn't just ignore it all and succumb to the desire for shut-eye.