Entrainment of circadian clocks in mammals by arousal and food

Circadian rhythms in mammals are regulated by clock cells, with the suprachiasmatic nuclei (SCN) acting as a master pacemaker.

• The SCN pacemaker can be reset by daily light-dark cycles, aligning internal rhythms with external time.
• Sensitivity to behavioral arousal during typical sleep periods can phase-shift the SCN and modify its response to light.
• Neural pathways from the brainstem and thalamus release neurochemicals that inhibit retinal inputs to the SCN or alter clock-gene expression.
• Circadian oscillators outside the SCN may respond to food intake, allowing the uncoupling of behaviors from light-dark cycles.
• The persistence of behavioral rhythms in mice with SCN clock-gene mutations indicates a potential diversity in the mechanisms of circadian regulation.

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