system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What actually determines the moment that you fall asleep?

Is it a specific mix of brain chemicals, heart rate, and environment conditions that trigger it? Is it more complicated than that? And on top of that, why is it sometimes easy to fall back asleep if you wake up, but sometimes you lie awake for an hour?
gpt
There really isn't a exact moment when you fall asleep, because it isn't a synchronised thing. Different parts of your brain do different things at different times. And sometimes you can be asleep, and awake at the same time, like sleep paralysis. And you can be asleep, without going through all the phases of sleep. And these stages are just based on our observations and scans, we don't actually know everything that happens in our brains. But we know that lack of sleep is deadly. There is fatal insomnia like  *fatal familial insomnia.*   


But since sleep messes up with our memory, we don't actually know when we fall asleep. Like for example when you get general anaesthesia, when they tell you to count down from 100, the moment you recall that things went "black" was just the moment your consciousness went asleep and memory failed, not the moment you fell asleep.  


Like... You know sometimes you just lay there, you feel like you are asleep, but you are awake, and next or when you get up, you feel well rested and fully awake? Well you did sleep, sorta, kinda, in a way. Only thing that really matters is that the certain parts of your brain did the things it had to do to make you feel rested. Human brains are really strange.