From Indiana Public Media’s “A Moment of Science:”
Tryptophan, commonly found in turkey, is an essential amino acid, meaning that the body cannot produce it and must get it from foods you eat. Tryptophan stimulates the production of serotonin, which is a chemical that helps keep people happy by calming anxieties, relieving depression, and promoting sleep.
But the tryptophan in a lot of foods competes with other amino acids to get into the brain. So you might actually feel more of tryptophan’s effect after eating a meal heavy in carbohydrates.
This is because carbs cause the body to secrete large amounts of insulin, which clears the bloodstream of most of the amino acids that compete with tryptophan. Thus, there’s more room for tryptophan in the brain and it ends up having a stronger effect. In addition, eating a large meal can stimulate gut hormones that cause a sleepy feeling unrelated to tryptophan.
This is the body’s way of ensuring that you’re still and quiet, so that it can better digest.