Figure 3.

Schematic representation of the change in melatonin levels over 48 h. The time of day at which peak melatonin levels are reached is the phase. The amount of time between two phases is the period. A change in the phase indicates a time shift in the circadian clock. A change in the period to more or <24 h indicates that the circadian clock is no longer being entrained. Time of day is denoted on the bottom. The light bars represent time subjects are normally awake and the dark bars represents a typical 8 h sleep time.