Tryptophan and turkey
According to popular belief, tryptophan in turkey meat causes drowsiness. Turkey does contain tryptophan, which does have a documented sleep-inducing effect as it is readily converted into serotonin by the body. However, tryptophan is effective only when taken on its own as a free amino acid. Tryptophan in turkey is found as part of a protein, and, in small enough amounts, this mechanism seems unlikely.
A more-likely hypothesis is that the ingestion of large quantities of food, such as at a Thanksgiving feast, means that large quantities of both carbohydrates and branched-chain amino acids are consumed. Like carbohydrates, branched-chain amino acids require insulin to be transduced through the myocyte membranes, which, after a large meal, creates a competition among the amino acids and glucose for insulin, while simultaneously creating tryptophan's reduced competition with other amino acids for the Large Neutral Amino Acid Transporter protein for transduction across the blood-brain barrier.