The rhythmic pattern of biological processes controlled by light over 24h is termed the circadian rhythm. Disturbance of circadian rhythm due to exposure to light at night (LAN) disrupts the sleep- wake cycle and can promote cardiovascular disease, diabetes, cancer, and metabolic disorders in humans. We studied how dim LAN affects the circadian rhythm and metabolism using male Drosophila. Wild-type flies exposed to the dim light of 10lux at night displayed altered 24h sleep-wake behavior and expression patterns of circadian rhythm genes. In addition, the flies became more vulnerable to metabolic stress, such as starvation. Whole-body metabolite analysis revealed decreased amounts of branched-chain amino acids (BCAAs), such as isoleucine and valine. The dim light exposure also increased the expression of branched-chain amino acid aminotransferase (BCAT) and branched-chain a-keto acid dehydrogenase (BCKDC) enzyme complexes that regulate the metabolism of BCAAs. Flies with the Bcat heterozygous mutation were not vulnerable to starvation stress, even when exposed to dim LAN, and hemolymph BCAA levels did not decrease in these flies. Furthermore, the vulnerability to starvation stress was also suppressed when the Bcat expression level was reduced in the whole body, neurons, or fat body during adulthood using conditional GAL4 and RNA interference. Finally, the meta- bolic vulnerability was reversed when BCAAs were fed to wild-type flies exposed to LAN. Thus, short- term dim light exposure at night affects the expression of circadian genes and BCAA metabolism in Drosophila, implying a novel function of BCAAs in suppressing metabolic stress caused by disrupted cir- cadian rhythm.