It's not the turkey — it's the carbs.

That might be the bottom line for why you'll slip into a short coma after your Thanksgiving feast this year, scientists say.

Before explaining why carbo-loading may be the culprit for the annual snooze, it's necessary to present a brief history of turkey and tryptophan.

Tryptophan is one of nine "essential" amino acids we use, meaning it can't be manufactured by our bodies but must be ingested in our food. It makes up about 1 percent of the protein in turkey — roughly the same proportion found in other meats and about half the level found in milk.

Because experiments showed that tryptophan could directly induce drowsiness in lab animals, it was blamed for several years for our post-Thanksgiving siestas.

The idea gained even more currency when a famous episode of "Seinfeld" showed Jerry and George feeding turkey and wine to Jerry's girlfriend so she would fall asleep and let them play with her collection of classic toys.

In more recent years, most stories on the topic have claimed that the turkey-sleep connection is "an urban myth," and that turkey would only make you sleepy if you ate it on an empty stomach.

The truth, it turns out, is probably more complicated than that, says Dr. John Fernstrom, a psychiatry professor and biochemistry researcher at the University of Pittsburgh.

Tryptophan does get into the brain and produces a neurotransmitter called serotonin, which can create feelings of satisfaction and drowsiness, Fernstrom said. And serotonin is a precursor for melatonin, the hormone that kicks in after dark and helps put us to sleep for the night.

On the other hand, the protein in turkey and other foods also contains a lot of other amino acids, some of which create feelings of alertness in the brain, so under normal circumstances, "if you're stuffing yourself silly with turkey, which would be a lot of protein, that by itself wouldn't raise how much tryptophan was going into the brain," Fernstrom said.

And that — drum roll, please — is where the Thanksgiving carbohydrates come into play.

When people load up on rolls, stuffing, mashed potatoes and other carb-heavy foods, it will spur a surge of insulin to make use of all that nutrition, and it turns out that insulin pulls almost every amino acid that's in the bloodstream into muscle tissue — except for tryptophan.

As a result, carbo-loading can leave a disproportionate amount of tryptophan in the bloodstream, and that could produce extra serotonin in the brain, thus inducing drowsiness.

The trouble is, Fernstrom said, the studies that show a link between heavy carbs and heavy snoring are not utterly convincing, and there could be several other reasons why the Pilgrims and the Indians all shared a nap.

For one thing, there is the psychological ambience of the holiday, said Timothy Maher, a professor of pharmaceutical sciences at the Massachusetts College of Pharmacy and Health Sciences in Boston.

"It's the national day of rest," Maher said. "It's not like you're rushing through your lunch so you can go back to work."

Then there are the other factors at work: the comfy recliner, the boring football game on TV and Uncle Andy explaining his latest business trip in excruciating detail.

Add to that the fact that there is a direct relationship between how full someone is and how satisfied and ready for a nap they are.

Meanwhile, a new area of research shows that another brain chemical called orexin may play a key role in the drowsiness cycle, regardless of whether any turkey is on the plate.

Scientists have long known that when people are hungry, they are more alert. In prehistoric times, that was probably a signal for people to get off their duffs and start stalking the latest meal, as opposed to simply taking a short hike to the refrigerator.

Recent studies have shown that animals' brains secrete orexin when they are hungry and their baseline metabolism is low. Conversely, after a meal, when baseline metabolism rises and the animal gets sleepy, the output of orexin drops off.

So whether it's a dropoff in orexin, a surge of tryptophan or a football game nobody cares about, eyelids are almost certain to droop after Thanksgiving, Fernstrom said.

"All of these signals end up telling the brain, 'I'm going to sleep now."'