Many attribute the sleepy feelings after a large Thanksgiving meal to the amino acid, tryptophan.
This amino acid is an essential amino acid, meaning that it cannot be produced by the body. Thus, we need to ingest it regularly though our food. In your body, it is used to manufacture niacin (a B-vitamin). This contributes to the production of serotonin, which can regulate sleepiness. That typical Thanksgiving focus, turkey, contains tryptophan, though not in a concentration any higher than most other meats.
It has become somewhat widely accepted that by ingesting turkey, the tryptophan is what causes a person to be lethargic. However, it is unlikely that this is the cause of the sleepiness. In truth, the competition that tryptophan goes through with all the other amino acids in the Thanksgiving meal causes it to have very little effect on the brain. It is more likely the alcohol consumed on this holiday that causes the tiredness.
However, if you have an empty stomach, it may have a greater chance for effect. So if you have leftover turkey and need a snack before bed, turkey may be a good idea for helping you drift off to sleep.