Every year I promise me I won’t eat myself into a food coma: I’ll eat responsibly, I’ll fill my belly with salad, and I’ll go light with the turkey and gravy. Instead, I wake up three hours after Thanksgiving dinner, slumped like Robinson Crusoe on the living room floor, under a pile of my nephews’ toys. My shirt is covered in light brown stains and greasy handprints stain my jeans.

What is it about Thanksgiving Does this send me – and millions of other Americans – into digestive oblivion? Are we all happy eating turkey, or is there another reason why Thanksgiving is the holiday of laziness?

You’ve probably heard that turkey meat is dripping with a sleep-inducing chemical called tryptophan. And while it’s true that this stuff plays a role in putting your brain to sleep, saying it does it on its own is like saying Neil Armstrong jumped to the moon by himself.

For one thing, turkey isn’t particularly loaded with tryptophan. Ounce for ounce, a roast chicken, a grilled steak or a rack of pork ribs all have comparable quantities. Freeze-dried tofu contains about twice as much tryptophan as turkey, and I doubt you’ll hear your Southern California cousin complaining about how sleepy he is after stuffing himself with fake meat.

Carbs are the real culprit of Thanksgiving sleepiness. Cast your heavy-lidded gaze on the side dishes. Mashed potatoes, cranberry sauce and pie are high in carbohydrates and load your blood with glucose, a sugar. To regulate the amount of glucose entering your muscles, your body releases insulin, which commandeers a set of amino acids to help you do your job. Tryptophan is also an amino acid, but it is not useful for glucose regulation. Instead, it is primarily used by the body to make mood-regulating hormones.

Normally, tryptophan has limited access to your brain because it is blocked by other amino acids. However, when called upon to help regulate glucose, tryptophan is in the clear. In the brain, it is converted to serotonin, then melatonin, known to cause drowsiness.

Turkey is not special. Any food containing a modest amount of tryptophan followed by about 30 grams of carbohydrates (a medium plate of spaghetti) will hijack the rest of your amino acids long enough to cause that feeling of brain fog. But the tryptophan/carbohydrate combo is only part of the reason for your torpor. What’s even more to blame is the fact that you’re eating. SO. Condemn. A lot.