system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that sometimes we go to sleep feeling hungry but in the morning that same feeling goes away?
gpt
The feeling of hunger is induced in your body by the secretion of the ghrelin hormone in the stomach. It does not mean that your body is out of fuel, it just means that your stomach was expecting some food and didn't get it. 

When you fast for a certain period of time, your body automatically stops demanding food and switches to burning the fat stores for fuel. If your body is "fat-adapted" (it becomes used to consuming your body's fat easily), the period of fasting required for this to happen is less.

When you're using your fat for fuel, you don't feel hungry anymore.