The human metabolism rises and falls throughout the day, in accordance with how you eat. When food enters your stomach, the digestive system revs up to a higher state in order to deal with what is coming in. After a few hours, the food will have moved on and your digestive system goes into a low state. The highs and lows affect your energy levels, so you feel energetic a short while after eating, and tired in the couple of hours before the next meal.
Certain foods can cause this to happen with more fluctuation than others. Foods with high levels of MSG, such as certain Chinese dishes, have traditionally offered a crashing sensation after a large meal. Other dishes, like turkey with its high levels of tryptophan, can easily make you seek out a nap. It is at these times, you should fight the urge to snooze and instead choose to walk to rev up your metabolism after a large meal.