Why do certan foods make us tired?

I have personally experienced this sleepy feeling after eating large meals containing high glycemic index. Turkey also contains what is known as L-tryptophan, in substantial quantities, it is a natural sedative. And although turkey may be one of the "offenders", I tend to believe that the ingestion of large quantities of food during holidays such as the Thanksgiving feast or Christmas is linked to the high intake of both carbohydrates and branched-chain amino acids. Turkey is often cited as the culprit in afterdinner lethargy, but the truth is that you could omit the bird altogether and still feel the effects of the overeating.

Like I said earlier, turkey contains 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. Niacin, in turn, is used to produce serotonin, a neurotransmitter that exerts 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.

The bottom line is that there is an amino acid in turkey which -- under proper circumstances -- can make you sleepy, but a more likely reason for the feeling of tiredness is that eating a lot of food encourages the body to begin to shut down to digest the food.

Sources: http://chemistry.about.com/od/holidaysseasons/a/tiredturkey.htm
Sources: http://en.wikipedia.org/wiki/Tryptophan