When someone says, "Oh, man am I sleepy...(yawn) I ate so much Turkey...it always makes me sleepy...", first of all, don't be rude. 

Save that for later when your Uncle Ed starts in with the conspiracy theories after he's had...however many he's gonna have. No. 

Just simply smile and say, "Oh yeah? Wow. You must have eaten a lot! Like, over 8 lbs, right?"

(I said don't be rude. I didn't say don't be sarcastic) 

They will probably think they did eat 8 pounds of turkey - which would be most of the bird - but they didn't. Actually I don't know how much turkey the average person eats on Thanksgiving. I know how much I eat. I know it's no where near 8 pounds! A single three-ounce serving (about the size of a deck of cards) of roasted turkey without the skin contains: Calories: 145. Protein: 25 grams. Fat: 4 grams. Turkey has slightly less tryptophan than chicken, and many nuts and seeds have more. 
You would need to eat about 20 servings of turkey to consume as much tryptophan as one supplement, which would be enough to feel it...A LOT!

Other factors that can make you feel sleepy after eating a large meal include: 
Digestion
Your body increases blood flow to your stomach to digest the large meal, which can lead to less blood to your brain. 

Portion size
The large portion of turkey you eat at Thanksgiving dinner contains more tryptophan, carbs, and calories than you normally eat. 

Other foods that are high in tryptophan include:
cheese, yogurt, eggs, fish, pork chops, pork tenderloin, skirt steak, ground pork, chuck steak, and roasted ham.

And they all sound delicious! 

Eat, drink, be merry! 
HAPPY BIRDDAY!