we feel hungry when our body need energy or shortage of energy in our body to perform different metabolic function in the body.for every activity we do need some energy which is fulfiled by eating food.
After need of body on demand of body we are supplying food also body act like there is lack of food or energy in the body it is because on eating immediately body start to produce alcohol which is sedative in nature. This is one of the reason and another reason is after taking food all blood or maximum blood is directed to receive the food for energy in the intestinal system and due to low blood supply in brain body feel sleepy.