Turkey contains tryptophan, an amino acid that serves as a precursor to serotonin and melatonin, which regulate mood and sleep.

However, the amount of tryptophan in turkey is not unusually high compared to chicken or other meats, and it does not cause drowsiness when consumed as part of a meal because it competes with other amino acids to cross into the brain. The real factors behind post-turkey tiredness are overeating a large holiday meal, which diverts blood flow to digestion; high-carb sides like potatoes and stuffing that spike blood sugar and insulin; and often alcohol, all amplifying fatigue. Tryptophan only promotes sleepiness significantly when taken alone on an empty stomach in specific doses.