Melatonin, a naturally occurring amide, is a hormone synthesized by the pineal gland from the amino acid tryptophan. Melatonin regulates the dark-light clock in our brains that governs such things as the sleep-wake cycle, body temperature, and hormone production.
Melatonin levels increase from evening to night and then decrease as the morning approaches. People with high levels of melatonin sleep longer and more soundly than those with low levels. The concentration of the hormone in our bodies varies with age - 6 year-olds have more than five times the concentration that 80-year-olds have, which is one reason young people have less trouble sleeping than older people.
Foods that offer high levels of tryptophan are excellent choice for a late night snack, including walnuts, almonds and dairy (warm milk).