Melatonin: “Nature’s most versatile biological signal” Melatonin is regarded as a hormone which regulates the circadian rhythm of sleep. It is synthesised from the amino acid tryptophan and then released from the pineal gland; the process is controlled by a biological clock in the suprachiasmatic nucleus located within the hypothalamus.1 An adult produces 20 – 60 micrograms of melatonin every 24 hours.1 The release of melatonin from the pineal gland is suppressed by ocular light at the retina.1 In a person with a normal sleep-wake cycle melatonin is released at night, typically beginning 14 hours after spontaneous awakening, i.e. at 9 pm in a person who wakes up at 7 am.2 Melatonin release therefore provides time-of-day information, i.e. onset of darkness, to various organs and tissues throughout the body. During a normal circadian cycle, melatonin reaches a peak concentration at approximately 2 – 3 am.1 Melatonin secretion then decreases and due to its relatively short biological half-life of 15 – 30 minutes is undetectable in the bloodstream by approximately 7 am.1, 3 People who work night-shifts have a delayed peak in melatonin secretion, the degree of which is influenced by their level of night-light exposure and number of nights worked.4 In people who live in extreme northern or southern latitudes the duration of melatonin secretion is increased during the longer winter nights.1 As children enter puberty the rhythm of melatonin release is delayed resulting in later onset of sleepiness and a later natural wake time.5 The secretion of melatonin decreases in later adulthood and by the age of 70 years it is reported that a person’s nocturnal melatonin concentration may be less than a quarter of what it was when they were younger.6