A circadian rhythm is roughly 24 hour cycle in the biological process which can be observed in living beings, including plants, animals, fungi and cyanobacteria. This cycle provides time of day and time of year information to the brain and body which can be modulated on a daily basis by external cues such as sunlight and temperature to prevail light and dark cycle.
Yuri Endo 2012
Scientific evidence suggests that disturbance in the circadian system (the melatonin cycle) may result in poor sleep quality, low productivity, seasonal depression, and immune deficiencies. Among many others, jet lag and bipolar disorder are also believed to be the repercussions of circadian rhythm disturbance.
Studies have also shown that light has a direct effect on human performance and well-being on account of the way it influences the circadian rhythms, especially the body's release of melatonin, a hormone that helps regulate our sleep cycle. In that sense, it is said that the use of melatonin help entrain the circadian clock to environmental cycles. In the USA, melatonin tablets are sold as dietary supplement which are available at convenience stores without prescription. Whereas in many other countries, it is available only on prescription or not at all. Above image shows the bottle of melatonin tablets that I take as a jet lag cure or simply when I have trouble sleeping at night. (As I felt mild hallucinogetic sensation, to say the least, rather than drowsiness from 5mm tablets, I am currently on 3mm and have not yet to try out 1mm.)