Seasonal Light Hours Modulate Peripheral Clocks and Energy Metabolism in Mice

With the exception of latitudes close to the equator, seasonal variation in light hours can change dramatically between summer and winter. Yet, investigations into the interplay between energy metabolism and circadian rhythms typically use a 12 h light:12 h dark photoperiod corresponding to light duration at the equator. Here, we hypothesised that altering seasonal photoperiod affects both rhythmicity of peripheral tissue clocks as well as processes involved in energy storage and utilisation. Male mice were housed at one of three photoperiods representing light hours in summer, winter and the equinox. Mice housed at a winter photoperiod exhibited an increase in the amplitude of rhythmic lipid metabolism and a modest reduction in fat mass and liver triglyceride content. Comparing melatonin proficient and deficient mice, we provide evidence that the effect of seasonal light on energy metabolism is largely driven by differences in the rhythmicity of food intake, but not melatonin. Our results show that seasonal light impacts energy metabolism in mice and suggest that these effects are partly driven by modulating the timing of eating. Our work sets a course to integrate seasonal light duration in future circadian biology studies. RNAseq of livers from 32 male high fat diet fed C3H/HeNCrl mice, sampled every 6 hours over one full cycle, living in either short light or long light photoperiod, n = 4 per photoperiod per timepoint.