because after taking the heavy meal circulation of blood increases towards the stomach for the digestion of heavy meal this causes less supply of blood to the brain therefore working of brain is affected and we feels sleepy.
Actually after having heavy food , our body feels that you had full stomach food and you need not to worry and there is no more tensions and you can relax and we feel sleepy.
Actually after having heavy food we should not sleep, we need to have walk for few minutes so that digestion processing takes place very well.
