Thanksgiving, a day comprised of great family gatherings, stories you probably never want to be told, and grandmothers who drink too much. After thanksgiving dinner everyone sits back and unbuttons their pants to let their gut hang out. Soon enough people are feeling tired, but it's not caused but what you may have heard.
The common understanding in households is that turkey makes you tired, specifically a chemical in the bird called tryptophan. While tryptophan is a sedative (i.e. it could induce sleep), its content in turkey is similar to that found in other meats and food items. According to the United States Department of Agriculture, turkey has 0.24g of tryptophan per 100g while beef has .23g/100g and pork has .25g/100g.
The main reason why most people feel lackadaisical after Thanksgiving dinner is due to the fact that so much is consumed in one sitting. From turkey to mashed potatoes to stuffing and all the other sides (let's not forget dessert either) there is a massive intake of food. Foods rich in carbohydrates release serotonin in the brain, a neurotransmitter involved in sleep. Another common feeling of drowsiness can come from the alcohol consumed throughout the day and during the meal.
So if someone brings up tryptophan at the table as the reason for their drowsiness, you can correct them... but be careful, elders usually don't like being proved wrong by a youngster!