So we have all heard about turkey making you sleepy due to the tryptophan in it. So I thought I would look it up and see how this works, apparently tryptophan produces the B vitamin niacin which triggers serotonin to act as a calmin agent in the brain, which helps you sleep. But it seems that while the tryptophan does induce sleep, it needs to be taken on an empty stomach to make you drowsy. I am guessing no one eats just turkey on Thanksgiving.
Instead of blaming the turkey there are other factors that play into why we feel so sleepy on Thanksgiving, overeating is a main one. It takes a lot of energy to digest and when we eat in such large quantities, because who really eats a small meal on Thanksgiving, it makes us tired. Fats also slow down the digestive system more than other foods so when you eat a lot of food that has a high fat content it slows down the digestive process, and therefore makes you tired. We also relax at the meal portion of the holiday, the stress of cooking is finally done and we can all enjoy the food, causing us to relax and get a little tired. It seems that the overeating, relaxing and large amounts of fat have a higher influence on making one sleepy on Thanksgiving than the turkey does.