Something I've been wondering

[jhos]

Can anyone tell me, how come during an afternoon nap you can enter straight into REM sleep but at nighttime you can't? How does your body know the difference? This has been bugging me for ages.

[Darkmatters]

It's because of REM cycles (I'll assume you know what those are, if not they're easy to google ). You know how they get longer with shorter breaks between throughout your sleep cycle, so that after like 10 hours' sleep you're almost in continuous REM? Somehow those cycles continue even when you're awake, so that if you go back to sleep you're already in REM, but a few hours after getting up for the day it resets.