What happens, basically, is that carbs trigger the release of insulin, which removes all the other amino acids from your bloodstream. When all of this goes down, tryptophan remains the sole amino acid in your blood. From there, it eventually moves into your brain, where it slowly morphs into the drowsiness-causing melatonin. This doesn't account for all the drinking you're probably doing before, during, and after you start masticating like a possessed, food-vacuum.
But the ultimate takeaway for Thanksgiving zombies out there: eating massive amounts of anything -- especially when coupled with alcohol -- is going to make you tired. Thanksgiving is a bonafide feeding frenzy, but there isn't some mysterious chemical that's solely to blame for your afternoon spent watching football with a beer resting on your gut.