Melatonin is a hormone manufactured by the pineal gland, which controls your circadian rhythm - that internal clock that determines your sleep/wake cycles. As the day turns to night, you secrete melatonin, start to feel drowsy and then fall asleep. After the age of 40, you tend to make less melatonin, which is why many older adults have a difficult time getting adequate shut-eye.
Consuming foods rich in tryptophan, such as turkey or milk, or the supplement 5-HTP can help as the body used this amino acid to make melatonin. Taking melatonin supplements on a regular basis can help some people get a good night's rest. It can also be an effective jet lag remedy, helping you adjust to the new time zone.
Melatonin is a strong antioxidant that seems to boost the immune system and may help evade heart disease. Claims of melatonin being an anti-aging miracle and cancer-fighting/prevention tool need more study.