The circadian system evolved under natural light-dark cycles, while modern humans spend much of their time indoors under electric lighting that differs substantially from daylight in intensity, spectral composition, and temporal structure. How such lighting environments influence the circadian system has not been systematically examined in a diurnal animal model under ecologically relevant conditions. In this study, we used the diurnal Nile grass rat (Arvicanthis niloticus) to assess daily locomotor rhythms across 4 lighting conditions designed to approximate common human exposure scenarios: rectangular daylight (D65‑R; ~5600 lux), semi‑sigmoidal daylight mimicking natural intensity dynamics (D65‑S; matched peak intensity with ~50% lower cumulative energy), fluorescent indoor light (F12; ~150 lux), and fluorescent light supplemented with a 1‑h midday daylight pulse (F12 + D65‑P). Using a within‑subject design (n = 8), male grass rats were housed under each condition for 2 weeks. D65‑R produced the highest daytime activity levels and the strongest day-night activity ratio, consistent with robust circadian entrainment. Despite matching peak intensity, D65‑S did not yield comparable circadian outcomes, indicating that cumulative photon exposure, rather than peak intensity alone, contributes to entrainment strength. Notably, the addition of a 1‑h midday daylight pulse (D65-P) partially increased circadian amplitude under otherwise fluorescent conditions, with higher periodogram amplitude relative to F12 alone. A separate cohort of males was exposed to D65-R or F12 for 6 weeks (n = 10/condition) to assess physiological outcomes, including metabolic and reproductive measures. Compared with the D65-R group, the F12 group showed a higher diabetic rate (10% vs. 40%) and reduced sperm motility (45 ± 6 vs. 19 ± 1%), consistent with potential downstream correlates of circadian rhythm disruption. Together, these findings demonstrate that lighting conditions characteristic of indoor environments produce weaker circadian organization than daylight-equivalent lighting in a diurnal rodent, which underscore the importance of light quality in shaping circadian rhythms and downstream physiological processes.
