Melanopic equivalent daylight illuminance of 2 lx maintains and restores physiological and neurophysiological circadian rhythms in rats

Understanding the non-visual effects of light is increasingly important, as circadian disruption from inappropriate exposure negatively impacts sleep, metabolism, and cognition. However, artificial lighting is rarely designed from a non-visual illuminance perspective, and its effects on biological rhythms remain incompletely evaluated. Here, we investigated three lighting conditions in rats: conventional light–dark (L/D), and two novel settings with matched visual illuminance but differing nighttime rat melanopic equivalent daylight illuminance (2–4 lx; rat mel EDI), enabled by tunable four-package white light-emitting diodes. Circadian rhythms were assessed using physiological (core body temperature, heart rate, locomotor activity) and neurophysiological markers (non-rapid eye movement sleep index, θ–γ cross-frequency coupling) across rhythmicity (periodicity, amplitude), stability (inter-daily stability, intra-daily variability), and state classification (Gaussian mixture modeling, k-means clustering). Relative to 4 lx, the 2 lx lighting most closely resembled L/D in minimizing rhythm loss and maintaining sleep–wake synchrony. Furthermore, after circadian disruption, switching to the 2 lx lighting restored physiological and neurophysiological markers. These findings indicate that spectrally optimized 2 lx lighting is a viable strategy to maintain or reinstate circadian health when nighttime illumination is unavoidable.