Responses to Intermittent Light Stimulation Late in the Night Phase Before Dawn.

The circadian clock is comprised of two oscillators that independently track sunset (evening) and sunrise (morning), though little is known about how light responses differ in each. Here, we quantified the morning oscillator's responses to 19 separate pulse trains, collecting observations from over 1300 at ZT23. Our results show that the advances in activity onset produced by these protocols depended on the tempo of light administration even when total exposure was conserved across a 15-min window. Moreover, patterns of stimulation previously shown to optimize the evening oscillator's delay resetting at ZT13 (an hour after dusk) were equally effective for the M oscillator at ZT23 (an hour before dawn), though the morning oscillator was by comparison more photosensitive and could benefit from a greater number of fractionation strategies that better converted light into phase-shifting drive. These data continue to build the case that the reading frames for the pacemaker's time-of-day estimates at dusk and dawn are not uniform and suggest that the "photologic" for the evening versus morning oscillator's resetting might be dissociable.