There’s a common belief that gets thrown around at this time of year: The reason you’re so sleepy after Thanksgiving dinner is because of the turkey. It’s true that turkey contains an amino acid called tryptophan, and tryptophan is needed to make serotonin, and serotonin makes you sleepy. But eating turkey alone doesn’t amplify serotonin production. Tryptophan is one of many amino acids that are flowing through your blood post-meal, and there isn’t enough tryp for it to affect serotonin—unless something prevents the other acids from getting to your brain.
That “something” is often the pie and other carbohydrates you snarffed down. According to Scientific American, a rush of carbohydrates encourages insulin secretion, and that prevents amino acids that aren’t tryptophan from getting to the brain. With no competition, tryptophan is free to make more serotonin, and that’s why you get tie-tie and need a widdle nap.