system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can humans go to bed hungry but wake up feeling completely full?
gpt
The feeling of being hungry is only partly related to the need for food. Feeling hungry is a sign a particular hormone is realeased. This hormone is released when you lack nutrients, sure (if your blood sugar levels are too low, for example), but mainly, it is released when your body expect some food to arrive soon, and is ready for digestion. So if you take your meals at regular times, and one day you skip one, about 30 minutes after your usual intake of food, you should feel very hungry. Because your body did not receive the food it was expecting.

But a few hours later, the levels of that hormone are down again (it's way past the time were food was expected), so you don't feel hungry. And you'll probably feel hungry again just before your next usual meal.

A very remarkable consequence of this is what happens when people do not eat anything for several days. The first two days, they'll feel very hungry. But after that, hunger disappears. At the opposite, people who would eat extremely little would still feel hunger for much longer, even though they get more food.