system
Given the following post, respond as if you're commenting on Reddit:
human
What happens when you sleep that makes you feel rested upon waking up?
gpt
It's poorly understood, but we think it involves the concentration of certain chemicals, including adenosine, in the cerebrospinal fluid.  
When you are awake, it builds up and makes you feel tired.  
While you sleep, it gets removed, leading to you feeling well rested when you wake up.  
  
Note that this is separate from the circadian cycle (body clock), which also affects how awake or asleep you feel based on a 24-ish hour cycle.