Oh, tryptophan is indeed linked with drowsiness - that's no myth. It's a biochemical precursor to serotonin, which has a calming effect on the brain and body. And tryptophan is indeed found in turkey meat. It's also present in chocolate, some fruits, dairy, red meat, and eggs.
However, tryptophan is almost certainly not the cause of Turkey Day food coma. First of all, the levels of tryptophan that we ingest in even a Thanksgiving-sized portion of turkey is not all that much more than is found in what we eat on any other day. Plus, tryptophan works best on an empty stomach, not a stuffed one!
The real culprit? It's probably a combination of your body working hard to digest a large meal, the effect of alcohol, and a fervent desire to put off doing the dishes.