This article was first published in The Skeptical Inquirer.
You may have heard the post-meal sleepies explained by the fact that turkey contains tryptophan, an essential amino acid that’s a biochemical precursor for serotonin and melatonin, two things that can make us feel tired. This is technically true, but some other facts can help put the turkey claims into context and better explain our need to nap after a bird-heavy meal.
First things first, turkey does contain tryptophan, but so do many other foods. Chicken, beef, lamb, and salmon all contain comparable amounts of tryptophan per gram, and both cheddar and parmesan cheese contain more. For reference, when tryptophan is given as a supplement to aid sleep, a typical dose is around four or five grams. A five-kilogram (eleven-pound) turkey, on the other hand, contains a total of about twelve grams of tryptophan, and one certainly does not eat five kilograms of turkey in one sitting!
So what is responsible for the overwhelming urge to take a nap after a big meal? The high fat and protein content of the food, the stretching of our intestines, and the blood glucose spike caused by eating dinner. You’ve probably heard of fight or flight, but have you heard of its lesser-known brother, rest and digest? After a big meal, our parasympathetic nervous system cues our bodies to increase salivation and digestion activities, as well as sexual ones. Couple that with a few glasses of wine, and you have a perfect recipe for a nap.
Anecdotally, I have been a vegetarian for around a decade now, and I can confirm that I still experience the overwhelming urge to snooze after a big meal such as Easter or Thanksgiving dinner.
