Melatonin

Helps increase the total sleep time (aspect of sleep quality) in people suffering from sleep restriction or altered sleep schedule (e.g. shift-work and jet lag).
Helps re-set the body's sleep-wake cycle (aspect of the circadian rhythm) and prevent and/or reduce the effects of jet lag/minimize jet lag (e.g. daytime fatigue, sleep disturbance) (for people travelling by plane easterly across two or more time zones/if flying east over two or more time zones).
Helps to speed up/reduce the time it takes to fall asleep (sleep onset latency aspect of sleep quality) in people who fall asleep slowly/with delayed sleep phase disorder.