This is only partly true. L-tryptophan in turkey does make you sleepy, but there are other reasons.
The turkey is often cited as the culprit in afterdinner lethargy, but the truth is that you could omit the bird altogether and still feel the effects of the feast. Turkey does contain L-tryptophan, an essential amino acid with a documented sleep inducing effect. L-tryptophan is used in the body to produce the B-vitamin, niacin. Tryptophan also can be metabolized into serotonin and melatonin, neurotransmitters that exert a calming effect and regulates sleep. However, L-tryptophan needs to be taken on an empty stomach and without any other amino acids or protein in order to make you drowsy. There's lots of protein in a serving of turkey and it's probably not the only food on the table.