<p>Artificial Light at Night (ALAN) has infiltrated many once-dark nightscapes, introducing a novel cue on biological clocks. Recent research has uncovered effects of ALAN on behavior, physiology, and fitness across taxa. However, questions remain about ALAN’s effects on key biological rhythm functions like diapause and sleep, and how disruptions to these rhythms can be linked to fitness declines. Further, most studies test effects of ALAN at the adult stage only, but little is known about the specific effects of ALAN across multiple life stages. We measured the outcomes of ALAN exposure on diapause termination (i.e., resumption of postdiapause development followed by adult eclosion), sleep-wake cycles, and fitness-related traits (i.e., survival, body size, and fecundity) in a seasonal insect: the walnut husk fly (<i>Rhagoletis juglandis</i>). We tested for life-stage specific effects of ALAN across four experimental groups: pupal ALAN exposure only, adult ALAN exposure only, pupal and adult ALAN exposure, and pupal and adult dark night exposure (control). Pupal exposure to ALAN delayed developmental timing; these flies eclosed later than dark-night controls. In adults, ALAN disrupted sleep-wake cycles regardless of pupal exposure, indicating an immediate behavioral response to light exposure. Notably, flies that were exposed to ALAN during the pupal stage only (i.e., not as adults) slept more during the transition from day to night compared to dark-night control flies. This reveals a lasting effect of ALAN on circadian regulation from pupal to adult stages. Despite these seasonal and circadian shifts, we found no fitness effects of ALAN. This work highlights the ability of ALAN to impact crucial functions like diapause and sleep, including through pupal exposure that has lasting effects into adulthood.</p>