Behavioral arousal stops light from shifting activity rhythms but not from triggering clock gene and immediate early gene responses in the hamster's daily rhythm control center

Confined hamsters showed reduced light-induced phase advances in wheel-running rhythms after exposure to novelty.

• Light exposure during the late subjective night was less effective in altering the timing of rhythms when hamsters were engaged in novelty-induced activity.
• Both Fos protein and Per1 mRNA were expressed in the suprachiasmatic nucleus (SCN) of all hamsters after light exposure, suggesting these molecules are consistently activated by light.
• The findings indicate that nonphotic stimuli may influence behavioral responses to light without affecting the transcription levels of clock genes.
• Different mechanisms may govern the effects of light and nonphotic stimuli on circadian rhythms, as they do not seem to interfere with each other's genetic regulation.

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