In human beings, L-tryptophan is the raw material the brain uses to make serotonin, melatonin, and niacin (vitamin B3). All three by-products of L-tryptophan metabolism can help you enjoy a good night's sleep. L-tryptophan thus plays a key role in helping regulate sleep-wake cycles in the brain.

Over twenty years ago, researchers at MIT demonstrated that when people eat a protein-poor, high-carbohydrate meal (such as spaghetti with tomato sauce), L-tryptophan from carbohydrate-rich foods goes right to their brains. The researchers discovered that compared to other amino acids, L-tryptophan has a unique advantage: Carbohydrates cause insulin secretion by the pancreas; insulin then clears all other amino acids from the blood that compete with L-tryptophan for entry to the brain, leaving only L-tryptophan, which is then shuttled across the blood-brain barrier into the brain. Once in the brain, L-tryptophan is converted to serotonin, giving the pasta lover a sense of satiety, a feeling of well-being, and the ability to drift off to dreamland (many antidepressant drugs also work by increasing levels of serotonin in the brain).

Dietary L-tryptophan increases the amount of REM (rapid eye movement) sleep, a stage of sleep that involves dreaming and deep rest. It also stimulates the production of melatonin, a neurohormone that helps control sleep-wake cycles. When too little L-tryptophan gets into the brain, melatonin synthesis declines, leading to the increased probability of nightmares and, consequently, a poor night's sleep (melatonin, in turn, causes the release of a brain chemical called vasotocin, which is also involved in REM sleep). Since melatonin secretion tends to decline with age, it's not surprising that elderly people have more trouble falling asleep and sleep less than they did in their youth.

Most people eat 1 to 1.5 grams per day of dietary L-tryptophan. Two to four grams of L-tryptophan have been used clinically to reduce the time required to fall asleep when taken one to two hours before retiring, but even 500 milligrams (one-half gram) of L-tryptophan can induce sleep. Because of its scarcity in food and severe competition with other amino acids for entry to the brain, it is difficult to ensure that the intake of L-tryptophan is high, even when protein intake is low. Stress, which makes it difficult to fall asleep, complicates the matter because it tends to lower levels of L-tryptophan in the body (Cortisol, a hormone released in response to stress, is responsible for this effect). Stress thus creates a vicious cycle of sleeplessness by depleting the very amino acid that the brain needs to fall asleep.

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