Figure 1.

Formal properties of circadian rhythms. A. Schematic illustrating parameters commonly used to quantify circadian rhythms. A sine wave is used to represent the output of the rhythm (e.g., PER2 expression in the SCN). Gray and black bars on the abscissa represent subjective day and subjective night, respectively. B. Schematic illustrating changes in circadian period and waveform. C. Schematic illustrating photic phase resetting. A light pulse during early subjective night causes a phase delay (blue) and a light pulse during late subjective night causes a phase advance (green).