The common experience of sudden fatigue following a large meal, often called the “Turkey Coma” or post-Thanksgiving slump, is a familiar phenomenon. This drowsiness, scientifically known as post-prandial somnolence, has long been attributed to the poultry at the center of the meal. The widespread assumption is that turkey contains a unique substance that sends the eater straight to sleep. This article investigates the science behind post-meal fatigue to determine if turkey truly is the culprit.
The Tryptophan Theory: Why It’s Blamed
The popular theory focuses on L-tryptophan, an amino acid found in turkey meat. Tryptophan is essential, meaning the human body cannot produce it and must obtain it through diet. It is a precursor to several compounds, including the B-vitamin niacin and the neurotransmitter serotonin.
Serotonin regulates mood and is directly involved in the sleep-wake cycle. The body uses serotonin to produce melatonin, a hormone that signals the brain it is time to sleep and helps regulate the circadian rhythm.
Because of this metabolic pathway, people hypothesize that consuming tryptophan-rich food, like turkey, floods the brain with the raw materials needed to produce sleep-inducing chemicals. This connection creates a simple, yet largely inaccurate, explanation for post-meal sleepiness.
Tryptophan and the Blood-Brain Barrier: Why Turkey Isn’t Unique
The idea that turkey is uniquely high in tryptophan and solely responsible for sleepiness is a misconception. Turkey contains a concentration of tryptophan comparable to or only slightly higher than other common protein sources, such as chicken, beef, and cheese. A serving of chicken breast, for instance, contains a similar amount of the amino acid as a serving of turkey breast.
Tryptophan must cross the blood-brain barrier (BBB) via a complex transport mechanism to affect the brain. The BBB is a highly selective protective layer controlling which substances pass from the bloodstream into the central nervous system. Tryptophan must compete with about five other large neutral amino acids (LNAAs), including tyrosine, leucine, and valine, for limited transport carriers to cross this barrier.
When a person eats a protein-rich food like turkey, the blood is flooded with all these competing amino acids simultaneously. Tryptophan is one of the least abundant amino acids in most proteins, so it is often outcompeted by the others. Consequently, when turkey is consumed alone, only a small fraction of its tryptophan content reaches the brain, making the effect on sleep negligible.
The General Science of Post-Meal Sleepiness
The phenomenon of feeling tired after eating, known as post-prandial somnolence, is a complex physiological response that occurs regardless of turkey consumption. This process involves a shift in the body’s autonomic nervous system toward the “rest and digest” state. When a meal enters the stomach and small intestine, parasympathetic nervous system activity increases, promoting digestion and resulting in lower energy.
Digestion is an energy-intensive process requiring the body to redirect blood flow to the gastrointestinal tract. This increased circulation facilitates nutrient absorption in the stomach and intestines. Although it was once thought this redirection starved the brain of blood, research suggests that the body’s tight regulation of cerebral blood flow ensures the brain’s oxygen supply remains stable.
Post-meal hormonal changes also contribute to drowsiness. For instance, the production of alertness-promoting hormones, such as orexin, may be inhibited after eating. Additionally, the gut can release sleep-promoting hormones and small protein molecules called cytokines, especially following a high-calorie meal, which are linked to fatigue.
The Role of Carbohydrates and Meal Size
The true drivers of exaggerated fatigue after a large holiday meal are the supporting side dishes and the sheer volume of food consumed. The typical holiday spread includes significant carbohydrates from items like mashed potatoes, stuffing, and pie. These carbohydrates cause a rapid spike in blood glucose, prompting the pancreas to release a large amount of insulin.
Insulin’s primary function is helping cells absorb glucose, but it also affects amino acids. The hormone drives most of the competing LNAAs out of the bloodstream and into muscle cells for protein synthesis. Tryptophan, however, is not taken up by muscle cells to the same extent as the others.
The result is a dramatic increase in the ratio of tryptophan to all other competing LNAAs in the blood. With less competition for transport carriers, a much larger amount of tryptophan can cross the blood-brain barrier and enter the brain. This synergistic effect—tryptophan combined with a massive carbohydrate load—enables the production of mood-regulating serotonin and sleep-inducing melatonin.
Exacerbating this effect is the simple factor of overeating, common during holiday feasts. A larger meal requires greater activation of the parasympathetic “rest and digest” system, leading to pronounced sluggishness. If alcohol is consumed, its sedative effects further contribute to the overall fatigue.