system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do you sometimes take a nap and wake up in an even worse mood?

Sometimes when you take a nap you wake up feeling rejuvenated and refreshed but sometimes you wake up feeling miserable and agitated...why?
gpt
You are probably "napping" too long, and actually falling asleep for a longer period of time. Rapid eye movement (REM) occurs about an hour and a half after you fall asleep initially, which when woken up in the middle of, will often cause grogginess and lethargy.

When taking a nap, you don't need to fall asleep; your aim ought to be to simply relax, clear your mind and let your body physically and mentally rest with your eyes closed. If you fall asleep, your body will jump into a highly taxing rejuvenation mode.

I believe the biggest "bang for your buck" kind of napping is about 20-25 minutes at a time, where you will get the most benefit for the time spent napping. More than that, you risk being groggy, still tired, etc. when you awake, and any less than that (e.g. 5-10 min.) might not be worth the effort as much.