Figure 5.A highly simplified diagram of phase shifts of the circadian system, as evidenced by changes in the melatonin rhythm itself, following oral treatment with fast release melatonin at different times. The maximum advance shift obtainable with a single treatment of 3-5 mg is approximately 1-1.5h. A combination of timed bright light treatment, timed melatonin and timed darkness/sleep applied over several days can produce much larger shifts. “Biological night” is the time of endogenous melatonin secretion and defines “circadian time” or CT which is independent of clock time. Individual timing of treatment according to clock time can vary substantially, as extremely early and extremely late phase people (larks and owls) will have differently timed “biological night”. After time zone travel, or a series of night shifts, circadian phase (or biological night) can be completely reversed with, for example, melatonin production during the daytime.