Regulation of circadian rhythms in mammals by behavioral arousal

Circadian rhythms in most mammals are synchronized to local time by phase and period resetting actions of daily light-dark cycles on a retino-recipient, light-entrainable circadian pacemaker, the suprachiasmatic nucleus (SCN). The SCN receives input from other brain regions, some of which mediate the phase and period resetting actions of behavioral arousal on circadian rhythms. We review historical milestones in the discovery of so-called "nonphotic" circadian clock resetting induced by environmentally stimulated arousal, or by feedback from clock-controlled rest-activity cycles. Topics include species generality, interactions between concurrent or successive photic and nonphotic inputs to the circadian clock, neural pathways, neurotransmitters, and clock cell responses that mediate resetting by behavioral arousal. The role of behavioral inputs to the circadian clock in determining the phase of entrainment to local time in natural environments is not well understood. Nonetheless, nonphotic effects are of sufficient magnitude to raise issues for the design of experiments in behavioral neuroscience (any procedure that is sufficiently arousing may alter the timing of circadian clocks that regulate dependent variables of primary interest). Nonphotic inputs to the clock may be exploited in strategies to reset or strengthen circadian rhythms in humans.