You've probably heard the claim that the reason you get sleepy after Thanksgiving dinner is because turkey is high in something called tryptophan. But what, exactly, is tryptophan and does it really make you tired? The connection isn’t quite so direct.
L-tryptophan (the full name) is an amino acid that's found in foods like turkey, chicken, meat, cheese, yogurt, eggs, and fish. Your body uses tryptophan and turns it into a B vitamin called niacin. Niacin plays a key role in creating serotonin, a neurotransmitter that's associated with sleep and melatonin levels (a hormone that controls your sleep-wake cycles). And that’s where the whole “turkey makes you tired” connection comes in.
But it’s not the whole story. You see, eating tryptophan doesn’t immediately impact serotonin levels. Tryptophan is just one of many different amino acids that are contained in foods like turkey. And all of those amino acids compete to get transported to the brain. Tryptophan is one of the least represented amino acids in those foods, which means that it gets shoved out of the way by the others.
That’s where this last piece to the puzzle comes in. There’s one thing that allows tryptophan to easily enter the brain: eating carbohydrates. Carbohydrates cause your body to release insulin, which removes all amino acids—except tryptophan—from your blood. That means that tryptophan has no competition and can enter the brain easily, boosting serotonin levels. So eating a snack that’s all carbohydrates will react with stored tryptophan in your body and give you a much bigger increase of serotonin. You can actually use this to your advantage by eating a light carbohydrate-centric snack before bedtime.
So does a Thanksgiving meal make you feel tired? Yes, but it probably has as much to do with the carb-loaded stuffing that you're eating as it does with the turkey (not to mention the large portions of food, in general!).