Eating doesn't give you energy right away, in most cases, and overeating often has energy that doesn't convert quickly.

Potato chips, meat, bread, rice, etc, are calorie dense, but full of complex carbohydrates and fats that need to be broken down into more simple sugars.

That process actually uses energy in the short term, before giving you a boost of energy later.

Simple sugars, like those found in juices and desserts, don't need to be broken down, and provide energy very quickly to the body.

So, what happens when you overeat? Your body shuts down to process the food, then has nowhere to go with the excess energy. That energy is then converted into fats and stored in the body for later.

Digestion requires energy. More eating = more energy. Your stomach focuses on digestion and the rest of you feels tired.

Your body releases a chemical called cholecystokinin that aids with digestion and gives you the feeling of being satiated. It's also found in the central nervous system and some believe that it may give you that sleepy feeling after eating.

It makes sense that your body would want to slow down to digest too because blood flow is being directed from your muscles and to your digestive tract.

Also after eating you have just started digesting the food but it hasn't been broken down enough to be utilized immediately. Simple sugars are broken down first, then the complex carbohydrates followed by proteins then fats.

Humans, like other animals, are not adapted to a world with ready access to abundant macro-nutrients. How your body reacts to a 32 ounce big-gulp and a large fries is not something that evolution has had an opportunity to apply selective pressure to.

[deleted]

Digestion takes energy. While you're digesting, you're tired. Once you've digested, you have energy.

Carbs, I think, make you sleepy.

I don’t know how that works, you need carbs for energy right?

I do know that when I was in my 20s I had energy all the time, no matter what I ate. Now that I am in my 50s everything I do makes me feel tired.