||BACKGROUND AND AIM: Non-alcoholic fatty liver disease (NAFLD) is a growing public health concern worldwide. With the progression of urbanization, light pollution is becoming an inevitable risk factor for NAFLD. However, the role of light pollution on NAFLD is insufficiently understood, and the underlying mechanism remains unclear. The present study explored effects of constant light exposure on NAFLD and elucidated its related mechanisms. METHODS: Thirty-two male SD rats were divided into 4 groups (n=8 each): 1) rats on a normal diet exposed to standard light-dark cycle (ND-LD); 2) rats on a normal diet exposed to constant light (ND-LL); 3) rats on a high fat diet exposed to standard light-dark cycle (HFD-LD); 4) and rats on a high fat diet exposed to constant light (HFD-LL). After 12 weeks treatment, rats were sacrificed and pathophysiological assessments were performed. Targeted lipidomics was used to measure sphingolipids, including ceramides, glucosylceramides and lactosylceramides, sphingomyelins and sphingosine-1-phosphates in plasma and liver tissues. RESULTS: In normal chow rats, constant light exposure led to glucose abnormalities and dyslipidemia. In high-fat fed rats, constant light exposure exacerbated glucose abnormalities, dyslipidemia, insulin resistance, inflammation and aggravated steatohepatitis. Compared to HFD-LD rats, HFD-LL had decreased plasma sphingosine-1-phosphate and elevated liver concentrations of total ceramides and specific ceramide species (ceramide d18:0/24:0, ceramide d18:1/22:0, ceramide d18:1/24:0 and ceramide d18:1/24:1), and which were associated with increased hepatocyte apoptosis. CONCLUSIONS: Constant light exposure causes dysregulation of sphingolipids and promotes steatohepatitis in high-fat fed rats.