system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why am I hungry before I sleep, don't eat, then wake up not hungry anymore?
gpt
Hunger isn't just the feeling of an absence of food in your stomach. Hunger is a sensation caused by your brain when it detects that your immediately available energy (a chemical called glucose) doesn't match up to how much it thinks you need for your current activity rate.

When you sleep, you aren't very active, and your body can get by on reserves of energy (a chemical called glycogen, stored in the liver) that take longer to convert into useful energy. Immediately after you wake up, you're still in a restful state, so your brain doesn't think it needs to tell you to get more food. 

Once you've gotten up and started your morning routine, you might find that you begin to feel hungry again as you become more active, and your brain starts to realise that you will soon be needing more energy. And then it's time for breakfast :)