The substance people usually blame is the amino acid tryptophan , which the body uses to make serotonin and melatonin, both involved in sleep.
What tryptophan does
Tryptophan is one of the building blocks of protein and is used to produce serotonin (affects mood and relaxation) and melatonin (helps regulate the sleep–wake cycle). Because turkey contains tryptophan, it has a reputation for causing drowsiness after a big meal.
Why turkey alone isn’t the culprit
The amount of tryptophan in a normal serving of turkey is too low by itself to knock you out, and many other protein foods contain similar or higher levels. The sleepy feeling after a holiday meal is more strongly linked to large portions, high-carbohydrate sides (like potatoes and stuffing), and alcohol , which together can make you feel tired.
