Irregular sleep patterns are associated with a wide variety of health problems. Melatonin is the hormone used by your body to help you fall asleep, and thus supplementation is seen as a way to help get regular sleep. This is particularly useful for people who engage in shift work or are jet lagged.
Melatonin may also ease symptoms of irritable bowel syndrome (IBS),[1] reduce tinnitus severity,[2] lower blood sugar levels in people with type 2 diabetes,[3] and result in small (perhaps negligible) amounts of weight loss.[4] There is ongoing research into melatonin supplementation as an adjunct treatment in cancer,[5] although human studies on the topic are limited.
Some demographics tend to have irregular melatonin production or metabolism in their body. Smokers may be less responsive to supplementation due to increased enzymatic breakdown of the molecule, and older people seem to produce less melatonin during night time. This may be why melatonin seems to have a more consistent effect in older people.[6]
Finally, one meta-analysis suggests that melatonin supplementation may modestly improve symptoms of depression, although studies on the topic vary widely in their conclusions. While melatonin is likely safe and potentially useful for depression, it isn’t clear that it will work in all cases.[7]