Turkey Makes You Sleepy
"The belief that eating turkey causes drowsiness because of its tryptophan content, especially associated with big Thanksgiving dinners."
Turkey does contain tryptophan, an amino acid the body uses to help make serotonin, but there's nothing special about turkey specifically: chicken, beef, and even cheddar cheese all contain comparable or higher amounts of it. There's also a biological catch that undercuts the whole theory: tryptophan has to compete with several other, more abundant amino acids to cross into the brain, and eating a protein-heavy meal like turkey actually makes that competition harder, not easier, meaning a turkey dinner shouldn't meaningfully raise brain tryptophan levels at all. The much simpler explanation for the post-dinner slump is the sheer size of the meal itself, plus the carbohydrate-heavy sides and any alcohol, a combination well known to cause drowsiness regardless of what protein is on the plate.