Melatonin is a natural hormone (neuropeptide) released from the pineal gland in the brain. Melatonin is known to play a part in regulating sleeping patterns in all living creatures; not just humans.
Specifically melatonin interacts with other hormones and maintains the body’s “circadian rhythm” – an internal body “clock” influences the onset of sleep.
When it turns dark, the body synthesises higher levels of melatonin; during daylight, the production of melatonin drops. Exposure to bright lights in the evening or not enough light during the day can hamper normal melatonin cycles. It’s well known that long haul jet travel and night-time shift working will tend to disturb melatonin cycles, causing sleeplessness.
There’s some evidence that melatonin levels vary with age. Young children and babies secrete the highest levels of melatonin, and this level drops as we age. Reduced levels of melatonin in older adults could explain why some people have sleeping problems ie going to bed and waking up earlier than when they were younger. This theory is not without some controversy though.