What is the chemical in turkey that causes drowsiness?

According to popular belief, tryptophan in turkey meat causes drowsiness. Turkey does contain tryptophan, which does have a documented sleep-inducing effect as it is readily converted into serotonin by the body. However, tryptophan is effective only when taken on its own as a free amino acid. Tryptophan in turkey is found as part of a protein, and, in small enough amounts, this mechanism seems unlikely.