Melatonin is a neuropeptide predominantly produced by the pineal gland. Melatonin is secreted in a distinct circadian rhythm - stimulated by darkness, inhibited by light and independent of sleep. The phase of the circadian rhythm is influenced by day length (increasing in amplitude in the winter and decreasing in spring) or artificial light.
The levels of melatonin in the body tend to decrease with age and low levels may result in sleep disturbances such as insomnia, poor immune function, depression and other mood disorders.
Due to its circadian rhythm, melatonin must be collected at midnight in the dark and again on rising (0600-0800).