Just in case you didn't know, the L in L-tryptophan stands for the relative stereo-chemistry of the amino acid. There are both L and D forms of all the amino acids, but only the L forms are ever incorporated into proteins (no one knows why this is). L-Tryptophan is contained in many proteins out there, not just the ones in turkey. I don't know if it makes you sleepy, but I always heard that serotonin was responsible.