And lots and lots of food!
Did you know that when your stomach is empty, it's about the size of your fist? So really, you don't need to wolf down several plates of foods!
When you over-stuff yourself with chicken, mash potatoes, corn, ribs, barbeque pork, pie, cake, hamburgers, and wash everything down with sweet tea, what happens? Not only are you filling your stomach to its full capacity, but you're also causing food to back up into your esophagus. Ever felt food coming back up your mouth whenever you burp after a GOOD meal? Yep, that's food that couldn't fit into your stomach!
When your belly is overstuffed, then your body starts restricting blood flow to other parts of your body and starts increasing blood flow to your stomach. Your stomach needs energy to digest food, and the more food you have, the more energy your body needs. And since your body uses up so much energy to digest your food, you'll get sleepy.
Yes, this would explain why you get sleepy after a Thanksgiving meal. You overstuffed yourself! And we thought it was the tryptophan's fault!
When you eat, eat until you're satisfied, not until you're so stuffed that it hurts. For one, it makes you sleepy. Two, it just does your body no good.