system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does too much sleep cause headaches?
gpt
You sleep in cycles of REMs. If you complete a cycle you feel good even if you only slept like 5 or 6 hour. If you wake up in the middle of a REM cycle you feel tired. sleeping too much can make you wake up before completing cycles because you have had enough sleep already. Also lying too down the whole night inhibit blood circulation.