Pineal melatonin, an endogenous signal of darkness, is believed to be an important regulator of circadian and seasonal rhythms. The changing melatonin levels serve as hands of a bio-clock and dates of the biocalendar in vertebrates including humans. Circulating melatonin regulates and influences the sleep wake cycle, sexual development, as well as various immune, endocrine, and metabolic functions. Initially it was thought to be produced exclusively by pineal gland. Subsequently, it was shown that melatonin is also produced in several other tissues. Substantial amounts of melatonin are found in the gut as well as the brain. Moreover, melatonin is one of the most powerful scavengers of free radicals. Sleep disorders and low melatonin levels are frequently observed in people with autism spectrum disorders (ASD). The role of abnormal melatonin biosynthesis in the gastrointestinal system relating to the development of ASD symptoms warrants further studies. The important multiple role of melatonin in prevention and amelioration of ASD symptoms is not fully recognized at present. Melatonin appears to be promising as one of the efficient and seemingly safe adjunctive treatments in children and adults with ASD.