Melatonin production
Melatonin (N-5-methoxy-N-acetyl-tryptamine) is synthesized from tryptophan after it has been converted to 5HT _[17].

Its synthesis can be increased by oral tryptophan or vitamin B6, a coenzyme in tryptophan metabolism. Little melatonin is stored in the pineal gland, but it is synthesized and secreted in pulses in response to noradrenaline release at synapses within the pineal gland. Its release is increased by selective serotonin re-uptake inhibitor (SSRI) antidepressants and antipsychotic drugs, but is inhibited by caffeine, beta blockers, benzodiazepines and non-steroidal antiinflammatory drugs. It is also produced in the retina and gastro-intestinal tract.

Endogenous control
Without any exposure to light the endogenous rhythm of the SCN leads to a circadian rhythm of melatonin secretion of around 24.2h by the pineal gland.

Melatonin is a marker of the circadian clock function, but it also acts as a hormone of darkness to reinforce the synchronization of sleep with the light–dark cycle of the environment. The pineal gland in effect acts as a transducer, converting a photo-period signal into a chemical signal.