Minireview: Entrainment of the Suprachiasmatic Clockwork in Diurnal and Nocturnal Mammals

Daily rhythmicity is controlled by a master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus.

• The clock gene Per1 in the SCN peaks at midday in both nocturnal and diurnal species.
• Timing of light sensitivity is similar during nighttime in both species categories.
• Exposure to darkness can reset the SCN clock during the resting period at different circadian times for diurnal and nocturnal animals.
• Nonphotic stimuli like scheduled exercise and food shortage can shift the master clock and influence light synchronization.
• Arousal-independent factors, such as melatonin, have similar shifting effects in both nocturnal and diurnal rodents at the same circadian times.
• Arousal-dependent factors, like serotonin, induce phase shifts only during resting and have opposite effects on photic resetting between species.

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