Somewhere around the late 1990s, blaming tryptophan consumption for post-Thanksgiving lethargy became as much of a holiday tradition as the food itself. This amino acid — present in turkey meat — is supposed to accumulate in your brain, prompting your body to use it to make extra serotonin, a chemical that can make you feel sleepy.
But, as this RiskBites video and a very excellent article by Matt Shipman both explain, there's actually a simpler explanation for the sleepiness you feel after eating two helpings of turkey, trimmings, and pie. Simply put: You ate too damn much.
Many people gorge themselves at the Thanksgiving table. During the resultant digestive process, the body diverts as much as 50 percent of its blood to the small intestine, to maximize absorption of calories and nutrients. That means there is less blood available for physical activity. Furthermore, most traditional Thanksgiving meals are high in fat and protein content, which actually slows down the digestive process. So your body is going to be diverting blood to the small intestine for a longer period of time.
Unfortunately, pointing this out to the family members sprawled on your living room furniture will not make you sound nearly as clever as the tryptophan myth does. And the bit about the blood flow to the intestine might get you (slowly, sluggishly) lynched. Maybe, instead, you can just recommend a nice walk.
Read the rest of Matt Shipman's piece on tryptophan, including an interaction between mashed potatoes and turkey that could, conceivably, cause tryptophan-induced sleepiness.