system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can I be really hungry at night, go to sleep, wake up, and then not be hungry anymore all with out eating anything?
gpt
When sleeping your body requires less energy than when you are awake. 

Your liver stores glucose from the food you eat and can release it when required.

This release of glucose from the liver powers your body while sleeping, and your metabolic rate lowers.

When you wake up in the morning, until you eat breakfast, your liver can continue to release glucose at a higher level to keep you going. 

When you eat your first meal of the day your metabolic rate increases and your body starts to store glucose for the next time it is required.

tl;dr glucose and a lower metabolic rate while sleeping.