Phase-response curve to light and to exogenous melatonin

The y-axis shows the direction and relative magnitude of the phase shift produced by the administration of light (blue line) and melatonin (red line) at various times that are shown on the x-axis. The two x-axes show the time of melatonin administration or bright light exposure in either clock time (upper x-axis) or circadian time of hours before or after the dim light melatonin onset (DLMO) (lower x-axis). The rectangle represents sleep period, the reversed triangle represents the core body temperature nadir, and the arrow represents the DLMO. The clock time axis shows the DLMO at about 9 PM, sleep from about 11 PM to 7 AM, and the core body temperature nadir at about 4 AM. These represent the typical times and phase relationships among these rhythms when the circadian clock is entrained to a 24-hour day. For individuals with earlier or later circadian rhythms, the time axis should be adjusted accordingly. The phase response curves show the phase shifts to light or melatonin administration, but in a particular situation the overall 24-hour light-dark pattern, which includes the dark time during sleep, is important in determining the direction and magnitude of the resulting phase shift. The magnitudes of the phase shifts in the light and melatonin phase response curve should not be directly compared to each other as the magnitude will depend on the “dose” used and the magnitude of the shift of sleep/dark. (Reproduced from: Burgess HJ, Sharkey KM, Eastman CI. Bright light, dark and melatonin can promote circadian adaptation in night shift workers. Sleep Med Rev 2002;6(5):407-20. With permission from Elsevier.)