Although the passage of food into the gastrointestinal tract results in increased blood flow to the stomach and intestines, this is achieved by diversion of blood primarily from skeletal muscle tissue and by increasing the volume of blood pumped forward by the heart each minute. The flow of oxygen and blood to the brain is extremely tightly regulated by the circulatory system and does not drop after a meal, and is not a cause of post-meal sleepiness.
A common myth holds that turkey is especially high in tryptophan, resulting in sleepiness after it is consumed, as may occur at the traditional meal of the American holiday of Thanksgiving. However, the tryptophan content of turkey is comparable to chicken, beef, and other meats and does not result in higher blood tryptophan levels than other common foods. Certain foods, such as soybeans, sesame and sunflower seeds, and certain cheeses, are high in tryptophan. Although it is possible these may induce sleepiness if consumed in sufficient quantities, this is not well studied.
When alcohol is consumed with a meal, this may contribute to sleepiness after the meal, but is a nonspecific response to alcohol consumption and can occur independent of eating.