Serotonin is a neurotransmitter released by neurons in the brain. It's made from the amino acid tryptophan, which is a component of protein and which humans eating a normal diet consume in significant amounts. High levels of tryptophan in the blood signal the brain to make serotonin, which has many beneficial consequences on mood and affect, including promoting happiness, relaxation and the ability to get a good night's sleep.
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Feelings of Well-Being
Serotonin is sometimes thought of as a "happy" neurotransmitter. Where deficiencies in the molecule can lead to depression and fear, appropriate levels of serotonin secretion bathe neurons in the brain in chemicals that help promote feelings of comfort, contentment and well-being. In their book "Biochemistry," Drs. Reginald Garrett and Charles Grisham explain that the brain makes serotonin when it's signaled to do so. One signal is carbohydrate consumption, which explains why may individuals experience feelings of contentment or happiness upon eating a sugary or starchy snack. Serotonin is so important to happiness; in fact, patients with depression receive prescriptions to help the brain artificially increase serotonin levels, which relieves symptoms.
One of the benefits of serotonin is sleepiness, which, while not necessarily a benefit at the wrong time of day, is certainly welcome at night. A 2010 study in the scientific journal "Sleep" notes that if the brain's serotonin transporters are misshapen, patients experience insomnia. Further, individuals who have difficulty sleeping have long been advised to try a glass of warm milk before bed. Milk, like many sources of protein, contains the amino acid tryptophan, which signals the brain to make more serotonin and which may produce feelings of sleepiness. There is some speculation that the familiar "Thanksgiving Day coma," wherein people feel tired after a big meal, may be due at least in part to high levels of tryptophan in turkey meat.
A "feel-good" hormone, serotonin helps people to feel relaxed and contented. Macalester College notes that studies tie low serotonin levels to anxiety; like depression, chronic anxiety may be treated with drugs that help increase levels of serotonin in the brain. Like many hormones and brain chemicals, however, serotonin functions best when it's at optimal concentrations. Normal levels of the chemical promote relaxation, but extra won't necessarily increase feelings of well being--it may just make the individual feel sleepy.