A hormone that plays a key role in the body's diurnal (night and day) rhythms. Produced by the PINEAL GLAND and derived from SEROTONIN, it acts on receptors in an area of the brain above the OPTIC CHIASMA, synchronising them to the diurnal rhythm. A pharmaceutical preparation of melatonin is licensed in the UK for short-term treatment of insomnia in persons aged over 55; additionally, clinical experience suggests that when appropriate behavioural sleep interventions fail, melatonin might help in reducing insomnia in children with visual impairment, cerebral palsy, attention deficit hyperactivity disorder, autism, and learning difficulties. It is also sometimes used before magnetic resonance imaging (MRI), computed tomography (CT), or EEG investigations.