Melatonin is a natural hormone that plays a role in sleep. Melatonin production and release in the brain is related to time of day, rising in the evening and falling in the morning. Light at night blocks its production. Melatonin dietary supplements have been studied for sleep disorders, such as jet lag, disruptions of the body’s internal “clock,” insomnia, and problems with sleep among people who work night shifts. It has also been studied for dementia symptoms.