It is a naturally occurring hormone:
Melatonin is a natural hormone that our bodies produce to trigger our sleep cycle, otherwise known as the circadian rhythm. It is not only responsible for making us feel sleepy but also for inducing those deep levels of REM sleep we all desire.
It is affected by light:
Blue light emissions like those from cellphones, TVs, or computers, inhibit production levels of melatonin. Exposure to artificial light, especially at nighttime, make it more difficult to fall asleep due to this.
Natural Production Decreases with Age:
Melatonin levels gradually decline as we age, this in turn affects our quality of sleep. Most older individuals have trouble staying asleep in the early hours of the morning because they have less melatonin in active circulation.