Via Marc Rust’s AMR:blog (which I found through Henrik Gemal’s Mozilla Related Blogs page), the Environment, Health and Safety Online group debunks the urban myth of tryptophan causing Thanksgiving-afternoon sleepiness. (Tryptophan is an essential amino acid present in turkey and commonly blamed for causing drowsiness.)
Sure enough, tryptophan was apparently a popular sleeping aid until about ten years ago. However, those effects were only acheived on an empty stomach. And, they’re are negated when consuming tryptophan as part of a meal:
Sorry to say, that if you’re looking for the sedative effect, it’s unlikely you'll get it from eating meats like turkey. L-tryptophan doesn’t act on the brain unless you take it on an empty stomach with no protein present. So it probably isn't at fault for the sudden drowsiness that hits right after the meal when the football games come on, and the dishes are waiting!
It’s more likely due to the combination of drinking alcohol and overeating — not just turkey, but also mashed potatoes, ham, creamed onions, cranberries, sweet potatoes, peas, stuffing (or dressing, if you prefer), carrots, bread, pies, and whipped cream — all of which have the effect of puling the blood away from your brain to help your digestive tract do it’s work, and the sugar/insulin effect. […]
So, if you’re prone to eating a bit too much on Thanksgiving (and who isn’t?), perhaps a cup of tea or coffee beforehand would counteract the effects.