Abstract
The central circadian pacemaker in humans is innervated by melanopsin-containing retinal ganglion cells (mRGCs), which signal light information over both long durations and large visual angles. Temporal summation is likely due to the extended post-stimulus firing of mRGCs. Coupling temporal summation with periods of darkness between light stimuli, to allow for chromophore regeneration or adaptation, explains our observation that pulsed light is far more effective in engendering changes in circadian timing than continuous light: 2 ms pulses of 1700–1800 lux light administered every 7.7 s for an hour generates nearly three-fold the shift in circadian timing as the same illuminance administered steadily for an hour. Separate from inter-stimulus interval timing, little else is understood about the relationship between light flashes and their impact on the circadian clock. Increasing illuminance of continuous light has a sigmoidal relationship with circadian phase shifting, but the relationship between the intensity of pulsed light and circadian phase change is unknown and will be determined in the current study. After two weeks of a regular athome sleep/wake schedule (established via actigraphy), participants enter the laboratory for two days of monitoring. Two hours after habitual bedtime, participants are exposed to an hour-long sequence of broad spectrum full field 2 ms flashes, spaced every 15 s and ranging from 3 to 3000 lux. Shifts in salivary melatonin onset between the evening before and after the light stimulus will be determined during constant posture protocols and the dose response curve for illuminance and phase change modelled.