system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I go to sleep hungry, but wake up feeling so full that I can't eat without forcing myself?
gpt
Overnight your body gradually depletes it's glycogen stores in the liver and muscles. It then transfers to the ketogenic energy system, activated at low insulin levels which runs primarily off fat stores. 

The difference between these energy systems is that insulin sequesters energy, pushing calories out of the blood stream into cells which can cause hunger. Whereas in a ketotic state your cells pull it in automatically.