Blood levels of melatonin are up to ten times greater at night than during the day. This high concentration of nocturnal melatonin led scientists to conclude that the production of this hormone signals to the rest of the body that it is time to sleep. Indeed, melatonin supplements have been used for decades to treat sleep-related problems, such as insomnia, sleep apnea, and jet lag. In the morning, when we perceive that it is light, melatonin secretion ceases, which stimulates the production of other hormones and hence other body activities to begin. This orderly daily rhythm is of prime importance to our physical condition, intellectual capabilities, and emotional health.
Animals change their behaviors such as reproduction, hibernation, and migration based on the season.
It is believed that animals perceive the change of seasons by the patterns of light and dark. The mechanism used to implement this behavior is by the secretion of melatonin. When daylight hours grow shorter during the autumn months, melatonin production in animals automatically increases, helping to make the almost constant sleep of hibernation possible. When daylight hours increase during the spring, the pineal gland secretes less melatonin, triggering a new pattern of physical activity. The ways in which these seasonal patterns affect human behavior is under intense investigation.