Melatonin - marker for circadin rhythms

Melatonin is a naturally occurring hormone that is key to sleep and the sleep-wake cycle in humans and animals. In the body, melatonin is produced by the pineal gland in the brain when night falls. The retina detects failing light, and the level of melatonin gradually increases, reaching its peak in the middle of the night in natural circumstances.

In the morning when plasma levels of melatonin decrease, sleep ends and wakefulness begins. Melatonin is not a sedative, but a sign of darkness or night time, and is a cue for innate night time behavior. For example, in nocturnal animals, melatonin stimulates wakefulness. In humans, melatonin induces heat loss, reduces arousal and related brain activity and delays production of cortisol, which increases blood pressure and blood sugar, in preparation for sleep.

Studies have shown that when administered during the day, melatonin causes feelings of fatigue and drowsiness like those experienced when naturally preparing for sleep. The rise in prevalence of sleep disorders associated with age is reflected by the decrease in melatonin levels that occur with age.