A millisecond parameter space for phase-shifting the circadian pacemaker with near-ultraviolet light.

Light-mediated circadian entrainment is a fundamental sensory process across taxa, yet the contribution of ultraviolet-A (UVA; 315-400 nm) wavelengths remains poorly understood. Here, we characterize the phase-shifting effects of brief UVA light flashes in Drosophila ananassae, a species with known sensitivity to short-wavelength light via ocular and extraocular photoreceptors. Using a parametric array of 62 UVA LED protocols varying in flash duration (8, 16, 120 ms), frequency (0.13-1.0 Hz), and irradiance, we quantified circadian phase shifts in the locomotor activity rhythm under constant darkness following stimulation in either the delay (ZT13) or advance (ZT23) zones. Young flies (2-3 days old) exhibited robust phase delays and advances that scaled with flash luminance and frequency. Dose-response modeling revealed increased phase-shifting efficiency and lower energy thresholds with decreasing flash duration, especially at ZT23. By contrast, aged flies (40-45 days) showed significantly attenuated responses, particularly to 8 ms pulses, indicating an age-related decline in UVA sensitivity. These findings suggest that the insect circadian system is finely tuned to the temporal structure of UVA input, with optimal entrainment possibly occurring in response to brief, intermittent stimulation. The marked reduction in responsiveness with age highlights a potential erosion of fast-acting photoreceptive pathways. Given the conservation of UVA-sensitive photopigments across animals, these results offer comparative insights into how temporal light encoding influences circadian regulation across life stages and taxa.