In mammals, circadian rhythms in peripheral organs are impaired when animals are maintained in abnormal environmental light-dark cycles such as constant light (LL). This conclusion is based on averaged data from groups of experimental animals sacrificed at each time point. To investigate the effect of LL housing on the peripheral clocks of individual mice, an in vivo imaging system was used to observe the circadian bioluminescence rhythm in peripheral tissues of the liver, kidney, and submandibular salivary gland in PER2::LUCIFERASE knock-in mice. Using this technique, we demonstrated that the majority of individual peripheral tissues still had rhythmic oscillations of their circadian clocks in LL conditions. However, LL housing caused decreased amplitudes and a broad distribution of peak phases in PER2::LUCIFERASE oscillations irrespective of the state of the animals' behavioral rhythmicity. Because both scheduled feeding and scheduled exercise are effective recovery stimuli for circadian clock deficits, we examined whether scheduled feeding or scheduled exercise could reverse this impairment. The results showed that scheduled feeding or exercise could not restore the amplitude of peripheral clocks in LL. On the other hand, the LL-induced broad phase distribution was reversed, and peak phases were entrained to a specific time point by scheduled feeding but only slightly by scheduled exercise. The present results demonstrate that LL housing impairs peripheral circadian clock oscillations by altering both amplitude and phase in individual mice. The broad distribution of clock phases was clearly reversed by scheduled feeding, suggesting the importance of scheduled feeding as an entraining stimulus for impaired peripheral clocks.
