system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is it that when i get 8-9 hours of sleep i wake up tired but when i get 3-4 hours i wake up energized?
gpt
When we sleep our brain goes through a cycle of activity. This cycle will vary between 80 - 120 minutes and may repeat 4 or 5 times during the night. The "deepest" part of this cycle is called REM (Rapid Eye Movement) sleep. This is the state usually associated with dreaming. We will enter REM sleep various times throughout the night, each time lasting longer than the last. 

If you wake up during REM sleep, you may feel more groggy than if you woke up in an earlier stage of the sleep cycle. If you don't have a regular sleeping schedule then your body may not regulate these cycles and you might wake up in the middle of one and feel "worse". Where if you wake up after 3-4 hours of sleep you may be waking up during a more natural part of the cycle.