system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Sometimes when i get maybe 2 -3 hours sleep i feel great when i wake up, but sometimes i get 8 - 10 hours sleep and feel like crap and have to go back to bed later because im tired. Why does this happen?
gpt
It has to do with your sleep cycles.  For the average person, a sleep cycle will last around 90 minutes.  You have light sleep, medium sleep, deep sleep, and REM sleep, and they all take place within the cycle.  When you wake up during REM sleep (at the tail end of the 90 minutes), you feel a lot better than when you wake up during a deep sleep.  So, even though you get less sleep, if you wake up at the right time, you will feel more refreshed.