It is a commonly held belief that L-tryptophan, an amino acid found in turkey, is a sleep inducer, which explains the sluggishness often felt after indulging in a large Thanksgiving meal. In fact, tryptophan is present in many foods, and the Thanksgiving stupor is more likely a result of overeating.
Tryptophan is abundant in all poultry, not just turkey. It is also found in other meats, cheese, milk, yogurt, fish and eggs. If the portions are moderate, most people feel no more sleepy after a turkey dinner than one made with chicken or fish. Any large meal, particularly if alcohol is included, can produce feelings of sluggishness and sleepiness.