tryptophan

There is a naturally-occurring chemical called skatole. Its claim to fame is that it is named after, and smells like, fecal matter. It's "synthetic" but also "man-made" and there's a revolting distinction in this case. It's also an additive used in cigarettes and strawberry ice cream (??). Do your "duty" and read this article.
It's irrelevant whether you like turkey or not, because someone is going to be handing you a plate of it very soon. And after the meal there's a good chance that you'll get sleepy. But will the cause of your drowsiness be the turkey, the booze or your brother-in-law's tedious commentary about his golf game? We explain.