The short answer: No.
Just reading the word ‘tryptophan’ in a holiday story may be enough to put you to sleep, or at least roll your eyes. But the science behind why the ‘tryptophan makes you sleepy’ myth is more interesting than you may think.
The myth goes like this: eating turkey increases tryptophan levels in the blood, which increases tryptophan in the brain. Tryptophan in the brain increases serotonin production, and serotonin can make you sleepy.
Tryptophan is an amino acid, one of the nearly two dozen that form the building blocks of protein. The first problem with the turkey sleepiness explanation is that tryptophan is present at about the same levels in all protein, so a slice of turkey at Thanksgiving is no more sleep-inducing by itself than a piece of chicken any other day of the year.
Any post-prandial doziness on Thanksgiving may be more to blame on that piece of pumpkin pie for dessert than the turkey, said Dr. Dick Wurtman, an MIT sleep researcher and professor emeritus.
That is because increasing tryptophan in the blood by eating turkey doesn’t necessarily increase tryptophan in the brain.
“The problem is when you eat the protein you’re not just increasing tryptophan,” Wurtman said. “You’re also increasing blood levels of half a dozen other amino acids, which compete with tryptophan for getting into the brain.”
Tryptophan is the rarest amino acid, Wurtman said, so when it tries to cross the blood-brain barrier, it has a lot of competition. Wurtman likens it to a crowd of people pushing their way onto the same train in the Tokyo subway.
“The doors open, maybe you have 10 or 15 people trying to get in, and they’re competing, they’re fighting with each other they’re not all going to make it,” Wurtman said. “Some of them are labeled tryptophan and more of them are not. So the more others you have competing, fewer of the tryptophan are going to get in.”
If you eat more protein, more of both the tryptophan people and other people will crush around the doors, increasing gridlock.
But if you eat a carbohydrate soon after eating a piece of turkey, say, for example, a slice of pumpkin pie, Wurtman said the scene changes.
Carbohydrates increase the secretion of the hormone insulin. Insulin encourages most amino acids to be taken into tissues and absorbed by the body, reducing their levels in the blood.
“But insulin doesn’t lower blood levels of tryptophan,” Wurtman said.
Back to that Tokyo subway analogy, the insulin from that pumpkin pie acts like a bouncer, clearing all the competition off the platform, and leaving just the people labeled ‘tryptophan’ standing.
“The number of other people fighting to get on the subway are going to go down,” Wurtman said, allowing more tryptophan to get on the subway, or cross the blood-brain barrier.
“Then some cells in the brain use the tryptophan to make serotonin and more gets released,” Wurtman said, “and that’s why carbohydrates make a lot of people sleepy, but not turkey, that doesn’t do it.”
Other potential causes of that Thanksgiving day nap: some research suggests that stretching the small intestine induces sleepiness. And a big meal will send more blood to the digestive system and divert it from other systems, like your brain and muscles.