Anyone who's promptly passed out after indulging in an especially large Thanksgiving dinner is familiar with the theory: tryptophan, an amino acid found in turkey, makes you sleepy. It's become conventional wisdom in recent years that this is an accepted fact of life, but is it actually true?

The short answer is...not exactly. L-tryptophan, as it's officially known, can also be found in everything from chicken and yogurt to fish and cheese, none of which are typically associated with sleepiness. Once ingested, tryptophan is converted into the B-vitamin niacin, which helps create the neurotransmitter serotonin. Serotonin plays a key role in melatonin levels and sleep itself, hence the apparent causal link between turkey and tiredness.

Young child making a funny face pretending to bite into a roasted turkey
Credit: skynesher/iStock

As you can tell from all those intermediary steps, however, the link between the two is far from direct. Plenty of other amino acids are present in turkey, and most of them are found in greater abundance — meaning that, when all those chemicals are rushing to your brain after your second helping, tryptophan rarely wins the race without help.

That assistance comes in the form of carbohydrates, so you can probably see where we're going from here. Eating carbs — which abound in Thanksgiving dishes like mashed potatoes and stuffing — produces insulin, which flushes every amino acid except one from your bloodstream: tryptophan. Thus, your post-Thanksgiving sleepiness is actually the result of a perfect storm composed of tryptophan, carbs, and the large portions typically associated with the holiday.

So when you're sitting down for dinner this Thanksgiving and someone mentions tryptophan, feel free to serve them a generous helping of knowledge with their turkey.

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