Background <p>Climate change increases exposure to ultraviolet-B (UV-B) radiation, which inhibits photosynthesis and causes DNA and cellular damage. While moderate UV-B exposure can enhance secondary metabolites, the impact of circadian timing on UV-B-induced toxicity and metabolic adaptation remains insufficiently explored, particularly in medicinal microgreens. <i>Achyranthes japonica</i>, a species with high pharmacological value, remains understudied at the microgreen stage.</p> Objective <p>This study examined how short-term UV-B irradiation affects the growth, pigment accumulation, and antioxidant capacity of <i>A. japonica</i> microgreens across different circadian cycles. The normalized difference anthocyanin index, a biochemical response indicator that is non-destructive and induced by UV-B, was also evaluated.</p> Results <p>Physiological and metabolic characteristics are affected by UV-B timing. During the day, UV-B promoted the accumulation of anthocyanin and phenolic compounds while preserving the efficiency of photosystem II and biomass. In contrast, nighttime UV-B exposure triggered photochemical damage but did not trigger pigment production. Continuous UV-B exposure increased anthocyanin levels but significantly reduced growth, consistent with the effects of cumulative stress. Antioxidant activity, measured by DPPH, ABTS, and FRAP, was highest with daytime treatment and lowest with nighttime exposure. A strong correlation was observed between NDAI and anthocyanin content and antioxidant capacity.</p> Conclusion <p>The circadian time of UV-B exposure significantly affects the balance between protective metabolite production and growth stability maintenance. Short periods of UV-B during the day result in the most favorable balance of protective metabolite production and growth stability. These results highlight the value of microgreens as a model system for studying the timing of radiation-induced stress.</p>