To understand the answer, it's important to know what happens in your digestive system when you eat. While you're chewing, your stomach produces the hormone gastrin, which triggers the production of the digestive juices that begin to break down your food. That food then moves into the small intestine as the gut releases the hormone enterogastrone, which regulates blood flow, among other things. Meanwhile, your pancreas releases insulin to promote the absorption of glucose from the carbohydrates in the meal. Insulin stimulates the transport of a variety of amino acids into the brain, including the infamous sleepy chemical known as tryptophan.
You might notice that food comas don't happen after every meal—just the indulgent ones. There are a few reasons for this: a meal high in carbohydrates triggers a larger spike in insulin, which makes more tryptophan enter your brain and turn first into seratonin, which makes you feel good, and then into melatonin, which makes you feel drowsy. Glucose from the carbs also appear to block brain cells called orexin neurons, which are responsible for keeping you awake and alert.
It should be mentioned, however, that tryptophan in food has a negligible effect when combined with all the other amino acids and hormones and macronutrients in a meal. Stop blaming the Thanksgiving turkey! It has less tryptophan than chicken, anyway. Plus, high-protein meals don't have the same sleepy effect, since protein tends to promote the release of more stimulating amino acids.
Food comas are sometimes unavoidable—nobody wants to watch their diet during a holiday feast—but if you want to reduce your chances of nodding off after dinner, there are a few ways to do so. Watch your portions and eat slowly so your body's hormones have time to balance out. Also, make sure you're eating a nutritious meal without too much starch or fat and with enough veggies and vitamins. Learn more about the science of eating in the videos below.