L-Tryptophan

For many organisms (including humans), tryptophan is an essential amino acid. This means that tryptophan cannot be synthesized by the organism and therefore must be part of its diet. Amino acids, including tryptophan, act as building blocks in proteins. In addition, tryptophan functions as a biochemical precursor for the following compounds:
Serotonin (a neurotransmitter), synthesized via tryptophan hydroxylase. Serotonin, in turn, can be converted to melatonin (a neurohormone), via N-acetyltransferase and 5-hydroxyindole-O-methyltransferase activities.
Niacin is synthesized from tryptophan via kynurenine and quinolinic acids as key biosynthetic intermediates.
The disorder Fructose Malabsorption causes improper absorption of tryptophan in the intestine, reduced levels of tryptophan in the blood and depression.
Tryptophan is a constituent of most protein-based foods or dietary proteins. It is particularly plentiful in chocolate, oats, bananas, mangoes, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, sunflower seeds, pumpkin seeds, spirulina, and peanuts. Tryptophan is also found in turkey at a level typical of poultry in general, contrary to some stories that seem to indicate that the tryptophan in turkey is responsible for after-dinner sleepiness at Christmas (and Thanksgiving in the USA). In reality, this particular effect is probably due to the much larger than usual amounts of food in general consumed on these occasions, along with of course fairly large amounts of alcohol!
Many people found tryptophan to be a safe and reasonably effective sleep aid, probably due to its ability to increase brain levels of serotonin (a calming neurotransmitter when present in moderate levels) and/or melatonin (a sleep-inducing hormone secreted by the pineal gland in response to darkness or low light levels).

Clinical research tends to confirm tryptophan's effectiveness as a sleep aid and for a growing variety of other conditions typically associated with low serotonin levels or activity in the brain, such as premenstrual dysphoric disorder and seasonal affective disorder. In particular, tryptophan has been showing considerable promise as an antidepressant alone, and as an "augmenter" of antidepressant drugs.
Because of its effects on brain chemistry, L-trypophan (and also its metabolite 5-HTP) should not be taken while taking other drugs that work on the brain, including sedatives, antidepressants and tranquillisers, unless doing so under medical advice.