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.