Hormone found naturally in the body. It is responsible for adjusting the body's internal clock, regulating the day-night cycles and the sleep-wake cycles. Darkness induces the body to produce more melatonin and when its concentration in the brain is increased, it sends signals to the body to prepare for sleep. In contrast, light slows down melatonin production sending a signal to the body to awaken. Some people who have trouble sleeping have low melatonin levels, so taking melatonin contribute to the reduction of the time taken to fall asleep and favours the rhythms of the sleep-wake cycle.