In the human body the mechanism is such that the oxygen we breathe in and the vital nutrients in the food that we eat are absorbed by the blood and supplied t the different organs. This job is accomplished efficiently by about 5 litres of blood that circulate in a normal adult.
The amount of blood supplied to each organ depends on the oxygen and nutrition requirement of the tissue of the organ and the importance of the function it plays at any given time.
Under normal conditions, the liver receives about slightly less than one-third of the blood that the heart pumps. Likewise, the kidneys get about one-fourth, and brain about one-sixth. The remaining goes to the muscles and other parts of the body. These quantities change depending on the body work and needs.
When we eat, the food needs to be digested and the nutrients absorbed by the blood. That is the focus of the work shifts to digesting the food and so more blood is sent to the stomach walls. Automatically the flow of blood to other parts of the body, including the brain, is reduced.
Only a few other very important functions like the functioning of the heart and lungs are continued. Once the blood flow to the brain is reduced, the body becomes lethargic and sleep sets in. So, better take a nap alter meals, it is only natural.