After Thanksgiving dinner, many people start to feel a little drowsy.
Turkey typically gets the blame.
It supposedly contains high levels of tryptophan, an amino acid that is sold in a purified form to help people fall asleep.
But turkey contains about the same amount of tryptophan as chicken, beef and other meats.
If Thanksgiving drowsiness is not about the main course, what is responsible?
It may have more to do with the side dishes.
To understand, we first need to digest a little food chemistry.
To start, we get tryptophan and other essential amino acids from all the protein in our diet, not just from meat. These amino acids swim through the bloodstream, nourishing our cells.
Brain cells convert tryptophan into a chemical called serotonin. This neurotransmitter helps regulate sleep and appetite and high levels of serotonin are associated with calm and relaxation.
But tryptophan and other amino acids can’t access brain cells on their own — instead, teams of proteins transport amino acids across the blood-brain barrier.
As it turns out, Thanksgiving side dishes probably make it easier for tryptophan to get inside the brain.
Mashed potatoes, stuffing and bread — as well as dessert — contain a lot of carbohydrates, which stimulate release of the hormone insulin.
Insulin encourages our muscles to absorb certain amino acids from the blood — but not tryptophan. So eating all those carb-heavy side dishes increases the amount of tryptophan in the blood relative to other amino acids, which means more tryptophan gets into the brain.
This eventually translates to higher serotonin levels, which probably contribute to Thanksgiving stupor.
However, this complex chain of chemical reactions is not the only reason people feel sleepy on Turkey Day
Studies have confirmed big meals of any kind make people drowsy. It takes a lot of energy to digest all that food. Also, during festive meals, many people enjoy a little beer or wine, making slumber all the more appealing.
And on top of it all, preparing such a large meal is physically exhausting, not to mention all the arguing — I mean, socializing — with extended family.
For Scientific American’s Instant Egghead, I’m Ferris Jabr.