Every year, I I promise myself that I won’t eat myself into a food coma: I will eat responsibly, stuff my belly with salad, and eat the turkey and gravy sparingly. Instead, I wake up three hours after Thanksgiving dinner and find myself sprawled out on the living room floor like Robinson Crusoe under a pile of my nephews’ toys. My shirt is covered in light brown stains and there are greasy handprints on my jeans.

What is it about Thanksgiving that sends me — and millions of other Americans — into digestive oblivion? Are we all happy about turkey or is there another reason Thanksgiving is the sloth holiday?

You’ve probably heard that turkey meat is dripping with a sleep-inducing chemical called tryptophan. And while it’s true that the stuff helps put your brain to sleep, saying it does that single-handedly is like saying Neil Armstrong jumped to the moon all by himself.

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

Carbohydrates are the real culprit behind Thanksgiving sleepiness. Cast your heavy-lidded gaze at the supplements. Mashed potatoes, cranberry sauce, and cakes are high in carbohydrates and load your bloodstream with glucose, a sugar. To regulate the amount of glucose entering your muscles, your body releases insulin, which requires a series of amino acids to help do the job. Tryptophan is also an amino acid, but is not suitable for regulating blood sugar levels. Instead, it is primarily used by the body to produce 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 levels, tryptophan is on the safe side. In the brain, it is converted into serotonin and then into melatonin, which is known to cause sleepiness.

Turkey is nothing special. Any food with a modest amount of tryptophan followed by about 30 grams of carbohydrates (a medium-sized plate of spaghetti) will distract the rest of your amino acids long enough to produce that brain fog feeling. But the combination of tryptophan and carbohydrates is only part of the cause of your fatigue. What’s more to blame is the fact that you’re eating. So. Damned. Much.