Here’s the answer to that question compliments of the folks at about.com:
Unless a microwave dinner is your idea of a Thanksgiving feast, you probably have had firsthand experience with the after-dinner fatigue that sets in after the meal. Why do you want a nap? To escape the dishes? Perhaps, but the meal itself plays a big part in the way you feel.
The turkey is often cited as the culprit in after dinner lethargy, but the truth is that you could omit the bird altogether and still feel the effects of the feast. Turkey does contain L-tryptophan, an essential amino acid with a documented sleep inducing effect. L-tryptophan is used in the body to produce the B-vitamin, niacin. Tryptophan also can be metabolized into serotonin and melatonin, neurotransmitters that exert a calming effect and regulates sleep. However, L-tryptophan needs to be taken on an empty stomach and without any other amino acids or protein in order to make you drowsy. There’s lots of protein in a serving of turkey and it’s probably not the only food on the table.
It’s worth noting that other foods contain as much or more tryptophan than turkey (0.333 g of tryptophan per 100 gram edible portion), including chicken (0.292 g of tryptophan per 100 gram edible portion), pork, and cheese. As with turkey, other amino acids are present in these foods besides tryptophan, so they don’t make you sleepy.
So while it might not be the turkey that makes you pass out at the end of the day, you probably don’t want to stuff yourself with too much of anything else either. It might just be that eating so much food can just wear people out!
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