system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do you often feel just as exhausted when you oversleep as when you under sleep?
gpt
Our brain goes through natural phases of sleep in cycles - to feel rested, your brain must go through 9-10 full cycles or so of the various stages of sleep (including REM which is when dreaming happens and the neural pathways are refreshed and lead to feeling rested). If your brain never enters into REM (a sleep disorder) or doesn’t enter REM often enough (sometimes happens as a result of sleep conditions - too hot, too cold, not dark enough, etc.) it could lead to feeling tired, regardless of how long you spend in bed asleep.