One feels sleepy after eating, particularly if one consumes a sweet dish or a bakery (STARCH)stuff this is said to be normal then.Having a sweet tooth after an evening meal leads your brain to make large amounts of the Serotonin and Neurotransmitter, that lulls people to sleep naturally at night.

Eating sweets or food made from flour eg: cakes or pastas, causes the blood sugar to shoot up higher than the normal. This causes the pancreas in our body to make larger amounts of insulin, which pushes the protein called Tryptophan from your bloodstream to your brain, where it is converted to serotonin which makes one feel sleepy and drowsy.
We can restrict our self from being sleepy after the meals by restricting the food high in sugar and flour. If ever one wants to be awake and alert,consume food that do not cause the blood sugar to rise,the best food to be alert is vegetable salads, nuts, seeds, meat, fish and chicken.
More over ,after eating, all the body's energy does the work of digesting the food thus there is heavy blood flow to the stomach to aid digestion,leaving you lethargic and you do not feel like doing anything but its good if you can take a walk after eating sometimes sleeping soon after eating slows your metabolism ,its good if you can keep awake for an hour at least after eating, even lack of exercise or an easy going lifestyle triggers sleep after meals.Do regular exercise to remain active ,movement is important in our lifestyle where as most of us do less manual work now a days .A person who is tired also feel sleepy but then that works like rest after exercise that is considered healthy ,where as sleeping because you have been a couch potato is unhealthy as it affects your life style and stamina .