system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: When falling asleep, how does your body know when you're taking a short nap (1-3 hours) vs actual deep sleep (6-8 hours)
gpt
The brain is more than just your conscious experience. Like right now, you're not consciously aware of it, but your brain has a goal of listening for your name being called (the cocktail party effect), and that's why you can hear your name even in a crowded, noisy room. 

So even though your state of consciousness is "asleep," that doesn't mean your brain is inactive. It's actually doing quite a lot during sleep; including listening to the environment (I can yell to wake you up). And it can have a goal of waking up soon. However, it doesn't always work. sometimes you oversleep. 

Finally, your body has a natural internal clock. If you sleep during the middle of the day to take a nap, your natural internal clock is going to want to wake you up soon anyway because it's your normal waking hours. Whereas at night (or whenever you normally sleep), your internal clock is ready for a longer sleep.