Conditioned fear suppresses light-induced resetting of the circadian clock.

The possibility that circadian rhythms can be modulated by emotional state is suggested by clinical evidence of altered physiological and endocrine rhythms in primary depression and related affective disorders and is supported by experiments in humans and laboratory animals showing that stress disrupts circadian rhythmicity. How emotional state might modulate circadian rhythms is not known. Here we report that induction of the emotional state of fear disrupts a process essential for stable entrainment of circadian rhythms, the resetting of the circadian clock by environmental light. A cellular correlate of light-induced clock resetting, expression of the transcription factor Fos in the suprachiasmatic nucleus of the hypothalamus (the circadian clock), and a behavioral measure of clock resetting, phase shifts in free-running activity rhythms, were found to be suppressed in rats exposed to light in a context made to induce fear by previous pairings with intermittent footshock. These findings show that fear disrupts a physiological process mediating light-induced clock resetting and suggest a mechanism through which emotional state could modulate circadian rhythms.