Tryptophan is an aromatic amino acid that is essential for human metabolism. As an essential amino acid, it must be obtain from the diet and cannot be synthesized de novo in the human body. Tryptophan is commonly obtained from meat in the diet. Each year, during the Thanksgiving holiday, I am reminded by many of my relatives that their sleepiness is completely due to the high levels of tryptophan in the turkey that is consumed. There is a legitimate scientific rationale to this belief. Tryptophan is a precursor to the neurotransmitter known as serotonin and the neurotransmitter like compound known as melatonin. Serotonin is involved in producing a calming, mood stabilizing, and euphoric feeling in humans, while melatonin is utilized as a sleep inducing compound by the human brain. Thus, consuming high levels of tryptophan could potentially cause a lethargic and somnolent state if it is quickly converted into serotonin and melatonin in the brain. But… is this really what is happening after eating Thanksgiving turkey to produce the sleepiness that many of us often feel? It is highly unlikely that eating turkey could provide enough tryptophan to produce this result. In fact, taking very large doses of tryptophan would be needed to produce this result. Even if we consumed several whole turkeys, we would not have enough tryptophan to induce somnolence. So, what is the cause of the sleepiness that we feel on Thanksgiving if the tryptophan in the turkey is not the culprit? It is likely a combination of the alcohol and the extra large portions that we consume with the turkey. Alcohol is a known sedative/ anxiolytic compound and causes central nervous system depression. In addition, the extra large portions of food require longer periods of digestion, which is more likely to occur if we are resting as opposed to running around busily performing activities.
See the following YouTube Video for an interesting explanation of this phenomenon: