Bright Light Shapes Diurnal Sleep-Wake Rhythms with Associated c-Fos Activity in the Anterior Paraventricular Thalamus in the Nile Grass Rat

Sleep profiles of the Nile grass rat, a day-active rodent that is widely used in research, have not been fully characterized. Sleep electroencephalograms were therefore recorded in male Nile grass rats maintained under 12 h light/dark (LD) cycles with different light intensities and spectra. A crepuscular elevation in wakefulness was observed under LD cycles of 150 lux white light. Moreover, a stepwise increase in daytime wakefulness and a reduction in daytime non-rapid eye movement sleep were observed at higher light intensities (300 or 1,000 lux). The amount of nighttime non-rapid eye movement sleep remained stable regardless of preceding light conditions, whereas delta electroencephalogram power was enhanced during the day and early nighttime under 1,000 lux LD cycles, suggesting homeostatic control of sleep quality. Although Nile grass rats have ultraviolet-sensitive photoreceptors, daytime coexposures of ultraviolet light did not affect daily sleep amounts or quality under 300 lux LD cycles. We then explored correlations between brain activity and sleep-wake levels or light intensities (150 or 1,000 lux) using c-Fos immunostaining in brains sampled at four different times of the day. c-Fos immunoreactivity in the hypothalamic suprachiasmatic nucleus-the central circadian clock-displayed the highest signal at light onset under 1,000 lux LD cycles. Furthermore, c-Fos immunoreactivity in the anterior paraventricular thalamic nucleus increased at dawn and dusk, and the midday signal was amplified by 1,000 lux light. These results elucidate light-dependent sleep-wake profiles in Nile grass rats and suggest the possible involvement of the anterior paraventricular thalamic nucleus in daytime arousal control.