Thanksgiving feasters take heart. Contrary to popular belief, turkey's tryptophan dose doesn't cause drowsiness. In fact, the substance could possibly aid in the treatment of depression and multiple sclerosis.

Purified tryptophan is a mild sleep-inducing agent. That probably spawned the idea that turkey and other foods heavy in tryptophan cause drowsiness.

But tryptophan can't get to the human brain in large amounts when ingested as part of a massive Thanksgiving feast—it needs an empty stomach.

"Tryptophan is taken to the brain by an active transport system shared by a number of other amino acids [the chief components of proteins], and there's competition among them—like a crowd of people trying to get through a revolving door," said Simon Young, a neurochemist at McGill University