The urban legend about how the tryptophan in turkey makes you tired is as much a Thanksgiving tradition as stuffing and cranberries. Like many long-told tales, there's a grain of truth amid a helping of misinformation.

Kate Zeratsky, a Mayo Clinic dietitian, sets the record straight and explains why tryptophan isn't the only reason you may want to grab a nap on Thanksgiving day.

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Thanksgiving turkey. It makes your mouth water. And it makes you tired, right?

Have you heard that there's tryptophan in turkey, and that makes you sleepy? Well, that’s true and false.

"Turkey is a nutritious food," says Zeratsky.

She says, yes, turkey is a source of tryptophan, but so is a lot of other food.

"Tryptophan is an amino acid found in protein-rich foods. So any food that has protein. It could be turkey or another meat. It could be dairy products, like milk or eggs," says Zeratsky.

So why do we end up needing a Thanksgiving day nap?

"It probably wasn’t so much the turkey as it was the quantity of everything you ate. Sometimes celebrations include alcoholic beverages, which also can make people a little bit sleepy," says Zeratsky.

The turkey, the trimmings, the toasts to health and happiness ...

"… put all that together, and you might need a nap," she says.

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