Turkey does not make you sleepy. While it does contain the chemical tryptophan, which the body uses for production of serotonin, tryptophan will not have a sedative effect on the human body unless taken on an empty stomach. Chicken, cheese and pork all also contain tryptophan, and yet for some reason nobody claims to become sleepy from these foods.
The sleepiness felt after a thanksgiving meal is the normal result of eating a large amount of food in a short period of time, particularly carbohydrate-rich foods such as potatoes and bread. A placebo effect from believing that you will become sleepy may also play a part of causing one to perceive greater sleepiness.
Happy Thanksgiving, everybody!
