Phase response curve illustrates the relationship between the timing of administration of a sleep phase affecting drug and the effect on the sleep phase. For example, melatonin applied before sleep tends to advance the sleep phase. When applied in the morning, it tends to prolong sleep and delay the sleep phase. When applied in the middle of natural night, it has no effect. This accounts for the sigmoidal nature of the phase response curve to sleep affecting drugs in humans (when plotted as: relative circadian time vs. phase shift magnitude).