One not-oft-mentioned thing melatonin seems to do (at least in my own experience) is that it cancels out the effects of earlier-consumed caffeine in preventing

One not-oft-mentioned thing melatonin seems to do (at least in my own experience) is that it cancels out the effects of earlier-consumed caffeine in preventing sleep. So if you're overtired and awake only because of caffeine, melatonin will put you out pretty quickly.