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 12/12-hour 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 1000 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 1000-lux LD cycles, suggesting homeostatic control of sleep quality. Although Nile grass rats have ultraviolet-sensitive photoreceptors, daytime co-exposures 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 1000 lux) using cFos immunostaining in brains sampled at four different times of the day. cFos immunoreactivity in the hypothalamic suprachiasmatic nucleus-the central circadian clock-displayed the highest signal at light onset under 1000-lux LD cycles. Furthermore, cFos immunoreactivity in the anterior paraventricular thalamic nucleus increased at dawn and dusk, and the midday signal was amplified by 1000-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.Understanding how environmental light shapes sleep-wake regulation in diurnal mammals is essential for developing experimental models relevant to human physiology and sleep disorders. This study provides a comprehensive EEG/EMG-based characterization of sleep architecture in the Nile grass rat, a diurnal rodent with strong potential as a model for seasonal affective disorders. Bright daytime light enhanced arousal, reduced NREM and REM sleep, and increased delta power, revealing intensity-dependent modulation of sleep quantity and quality. cFos mapping further identified the anterior paraventricular thalamus as a candidate node linking light exposure to arousal regulation. These findings position the Nile grass rat as a valuable experimental species for investigating the mechanisms underlying human circadian sleep-wake organization and its modulation by environmental light. Arvicanthis niloticus Significance Statement
