Figure 1.

Phase response curve to a single 6.5-hour episode of bright light in young adults. Phase shifts (in hours) of the plasma melatonin rhythm are plotted with respect to the circadian phase at which the center of the 6.5-hour light stimulus was presented. By convention, phase delay shifts (shifts to a later hour) are plotted as negative numbers, while phase advance shifts (shifts to an earlier hour) are plotted as positive numbers. Phase shift magnitude was determined by assessing phase before and after the light stimulus. Circadian phase of the light stimulus was defined relative to the midpoint of the plasma melatonin rhythm (defined as 22h) assessed just prior to the stimulus. Data from circadian phase 6–18 are double-plotted for better visualization. The open circle represents a subject whose phase shift was determined using salivary melatonin. The solid line represents a best-fit dual harmonic function to the data points. The dashed horizontal line represents the assumed 0.54-hour average phase delay drift of the human circadian pacemaker between the pre- and post-stimulus phase assessments. Figure reproduced with permission from Reference 42 [Khalsa SBS, Jewett ME, Cajochen C, and Czeisler CA. A phase response curve to single bright light pulses in human subjects. J Physiol (Lond) 2003; 549(Pt 3):945–952].