A Thanksgiving dinner plate. (Matthew Mead/AP)

It just wouldn’t feel like Thanksgiving without certain key features: a roasted turkey, pumpkin pie, too much traffic, some family drama, and, to cap it all off, the annual food coma. It’s a natural progression: After the final bites of dessert, everyone sluggishly wanders over to the sofa and either submits to or fights off a nap while the conversation turns to musings on how turkey makes you drowsy. Of course, your know-it-all aunt will confidently inform you that it’s because of the tryptophan.

Don’t blame the turkey

Your aunt, it turns out, is wrong. Let’s clear this up once and for all: It’s not the turkey that makes you tired. Yes, turkey contains tryptophan, an amino that produces a brain chemical called serotonin, known for making you feel relaxed and sleepy. But pork and chicken have about the same amount as turkey, and you don’t hear people complaining about passing out after a ham sandwich or chicken dinner.

The tryptophan connection

Turkey doesn’t give you the serotonin sleepies, because when you eat it — or pork or chicken, for that matter — you get a wide variety of amino acids along with tryptophan that all compete for the same transporter proteins needed to carry them from your bloodstream to your brain. Imagine a huge crowd of different amino acids jostling to get into the same little shuttle buses to carry them through the body. If tryptophan were the only kind present, enough of it would get to the brain to make substantially more serotonin; but with all the various amino acids fighting to get on the same tiny vehicles, tryptophan can only trickle in.

Here are some of the reasons you might feel as stuffed as the turkey after eating a big Thanksgiving dinner. (Reactions/American Chemical Society)

Sides and dessert: The real culprits

It turns out the real Thanksgiving nap-inducers have been hiding behind the turkey all along. Yes, that means you, sweet potato casserole, stuffing and double-crust apple pie. These sides and desserts are all rich in carbohydrates, which don’t contain tryptophan but clear the path for it to get to the brain fast. “Carbohydrates soothe and tranquilize,” says Judith Wurtman, author of “The Serotonin Power Diet.” “It’s a very clear-cut effect.”

That’s because carbohydrates boost insulin, which diverts most amino acids away from the brain to other cells, reducing competition for those transporter “buses” and effectively giving any tryptophan that’s already in the body a VIP pass. Eating protein blunts the production of seratonin, but, according to Wurtman, once you consume a ratio of 4 parts carbohydrate to 1 part protein, the blunting effect is overridden. In other words, that mound of mashed potatoes, plus the biscuits, the cornbread and multiple desserts, mean protein is outnumbered. Pass the pie, and a pillow, please.

Feast fatigue

Then there is the enormous volume of food eaten during the holiday meal. Studies show that a loaded stomach induces sleepiness. Also, when blood is diverted to the digestive tract it can’t be as effectively flowing in your brain and muscles. Not to mention the copious amounts of wine, beer and other spirits drunk during the Thanksgiving feast that induce grogginess.

Time to rest and digest

Holidays, as you well know, can be stressful, whether you are trying to prepare a meal worthy of a Norman Rockwell painting (not to mention your mother-in-law’s expectations) or fighting four lanes of red taillights to get to the dinner. When the meal is over and the pressure is off, that low-grade fight-or-flight stress response subsides and the opposite biological response kicks in. It’s called rest-and-digest, where your body naturally slows down and conserves energy for repair and processing nutrients.

After this year’s feast, with that many bona-fide excuses to doze off, why fight it? Go ahead; close your eyes. Just don’t blame the turkey.

Have a question for Krieger? Her next chat is Dec. 4 at 1 p.m. Submit your questions now and join us live.

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Krieger is a registered dietitian, nutritionist and author. She blogs and offers a biweekly newsletter at www.elliekrieger.com. She also writes weekly Nourish recipes in The Washington Post’s Food section.