Photic Phase Shifts of the Mouse Circadian Clock Are Defined by Scene Irradiance Across Differences in the Spatial Distribution of Light

Light resets the mammalian circadian clock, but whether this response is determined solely by integrated light intensity (irradiance) or is also influenced by the spatial distribution of light remains unclear. We tested whether high-contrast or low-spatial-frequency patterns modulate circadian responses in mice independently of scene irradiance. Circadian phase shifts were measured following exposure to spatially patterned illumination, including high-contrast checkerboards and spot stimuli positioned at the horizon or zenith. Across all experiments, redistributing light within the visual field did not measurably alter phase-shift amplitude when irradiance was matched between paired conditions. Circadian responses were well predicted by total scene irradiance whereas a metric based on the region of highest radiance provided a poorer account of the data. These findings are consistent with effective spatial integration of light by the mouse circadian system. Within the range of spatial and irradiance conditions tested, our results support irradiance as a simple and effective metric for predicting circadian responses across light environments with divergent spatial distributions.