Age and melatonin
Under the age of 3 months little melatonin is secreted and there is no variation with light exposure. A nocturnal secretion pattern then emerges and the peak nocturnal concentration rises to around 1400pmol/l at around the age of 3 years. This then declines, especially during puberty, to a level which is maintained until around 40 years of age before it falls further. The peak nocturnal level in adults is 250–500pmol/l and a total of 30 µg is usually secreted in each circadian cycle, although melatonin is often undetectable (less than 40pmol/l) during the day.

Light exposure and melatonin
Melatonin secretion is particularly suppressed with wavelengths of around 460nm.

Light exposure can either phase advance or delay melatonin secretion, according to its timing. Light exposure in the evening delays melatonin secretion, and darkness in the morning prolongs it and helps maintain sleep. This mechanism underlies the phase response curve of the circadian sleep rhythm to light.

Reduction of melatonin secretion at night is seen with low-frequency (50–60Hz) electromagnetic radiation from, for instance, electric cables, pylons and mobile (cellular) telephones. Exposure to light early in the morning induces an earlier onset of melatonin secretion the next evening. A brief light stimulus at night inhibits melatonin secretion temporarily.

Complete suppression requires more than 1000lux for more than 2 h, but as little as 100lux may halve melatonin secretion.