The most often-cited culprit is an amino acid, or building block of protein, called tryptophan. This is found in turkey, but it is found in higher concentrations in other foods, including: egg white, cod, soybeans, parmesan and cheddar cheese, pork chops, and (for the adventurous) caribou. One hardly hears of anyone complaining of excessive sleepiness after eating these foods. So what is to blame?
It may be the other things we pile onto our plates and into our goblets. Alcohol certainly can make us sleepy, but it's not the only culprit. It has been demonstrated that meals rich in carbohydrates -- such as potatoes, yams, breads, and countless other Thanksgiving foods -- trigger insulin release. This promotes the uptake by muscle of amino acids other than tryptophan. Lonely and unused trytophan builds up in the blood and is shuttled across the blood-brain barrier where it is converted into a neurotransmitter, or brain chemical, called serotonin. The serotonin is further metabolized into melatonin, which is a hormone with sleep-promoting qualities.
Fernstrom JD et al. "Brain serotonin content: increase following ingestion of carbohydrate diet". Science 174(13):1023–5.
Wurtman RJ et al. "Effects of normal meals rich in carbohydrates or proteins on plasma tryptophan and tyrosine ratios". Am. J. Clin. Nutr. 77(1):128–32.