system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come I do not feel hungry in the morning if I don't eat breakfast, but if I do, I find myself wanting something to eat or drink.

On top of this, I also find myself more tired if I have eaten something rather than if I haven't.
gpt
It's actually kind of simple really.

If you don't eat breakfast, you're body looks to other ways to begin fueling itself. If there's nothing in your stomach, it seeks out and uses glycogen (stored energy from food you've recently eaten) to run itself, and also to help your brain function properly. (Your brain gets the first go at any stored energy.)

Naturally, when this begins to happen, and even through there's no food in your stomach, your body stops reminding you that you're hungry, and gets right away to burning up any energy reserves (again, the 'glycogen').

The reason you're probably feeling tired is because -- and let's say you ate something carb-heavy, such as a bagel, or a stack of pancakes -- considerable amounts of blood is now beginning to flow to your abdomen, to help to begin processing all of the stuff you've just eaten. (Part of the reason why you feel 'sleepy' after a big meal.)

Your body telling you you need a drink, or wanting something to eat soon after breakfast, is basically just your metabolism speeding up and/or requiring hydration so as to move the process along more efficiently.

Hope I've helped. :}