According to Snopes.com:
linkyThe Big Sleep
Claim: Eating turkey makes people especially drowsy.
Example:[Collected via e-mail, 2002]
Here's a holiday dinner factoid that I've never been sure about — does turkey contain a natural sedative that makes you feel sleepy after eating a lot of it?
Origins: Whenever my husband and I find our cats collapsed in a heap on the bed, emitting loud kitty snores, we look to one another and say, "Someone must have slipped them some turkey." As widespread lore has it, something in turkey induces sleepiness, making those who partake of the bird unusually drowsy.
In this instance, lore almost intersects with science. Turkey does contain tryptophan, an amino acid which is a natural sedative. But tryptophan doesn't act on the brain unless it is taken on an empty stomach with no protein present, and the amount gobbled even during a holiday feast is generally too small to have an appreciable effect. That lazy, lethargic feeling so many are overcome by at the conclusion of a festive season meal is most likely due to the combination of drinking alcohol and overeating a carbohydrate-rich repast, as well as some other factors: Two other factors that contribute to the desire to sleep at the dinner table are meal composition and increased blood flow to the gastrointestinal tract. Studies have shown that a solid-food meal resulted in faster fatigue onset than a liquid diet. The solid-food meal also causes a variety of substances to jump into action that ultimately leads to increased blood flow to the abdomen. This increase in blood flow and an increase in the metabolic rate for digestion can contribute to the "coma."1
Those who still feel wary of turkey's purported sleep-inducing properties should find solace in the knowledge that many items we eat contain tryptophan. Milk, beef, and beans are among the foodstuffs which house this amino acid, and experts say the average serving of chicken or ground beef contains as much tryptophan as a serving of turkey does. If tryptophan were truly the sandman's henchman, we'd be falling asleep at the wheel on our way home from KFC or McDonald's.
Yet tryptophan may not be wholly innocuous. During the 1980s L-tryptophan was dispensed over the counter as a popular dietary supplement which buyers used for insomnia, appetite control, depression, premenstrual syndrome, stress reduction and other problems. But in 1989 the FDA recalled these supplements and urged the public to stop taking them immediately after they established a link between dietary supplements containing L-tryptophan and that year's mysterious outbreak of eosinophilia-myalgia syndrome (a painful blood disorder which can cause high fever, rash, weakness and shortness of breath, among other symptoms) in the United States. The EMS epidemic ultimately struck more than 1,500 people, killing at least 37.
The true culprit in that outbreak was never pinned down. At one point the disease appeared to be spurred only by the L-tryptophan supplements made by one particular company, leading to an "impurities gained during the manufacturing process" hypothesis. Yet this theory, though promising, did not adequately explain all instances of the disease. As to where things now stand, according to the FDA: Based on the scientific evidence that is available at the present time, we cannot determine with certainty that the occurrence of EMS in susceptible persons consuming L-tryptophan supplements derives from the content of L-tryptophan, an impurity contained in the L-tryptophan, or a combination of the two in association with other, as yet unknown, external factors.
The FDA does not currently prohibit the marketing of L-tryptophan.
People still feeling anxious about the prospect of tearing into a drumstick should consider that those who took the dietary supplement were, on average, ingesting 1,000 to 2,000 milligrams of L-tryptophan daily. Four ounces of turkey contain only about 350 milligrams of tryptophan, and (unlike people on dietary supplements, who take them every day) most folks don't ingest that much turkey every day of the week.