Impact of Day Length on Brain Glucose Metabolism in Men: A Large-Scale Repeated Measures PET Study

The brain is the most energy-demanding organ, yet whether cerebral energy homeostasis exhibits seasonal rhythmicity remains unclear. In this study, 432 healthy men underwent a health checkup program with fasting-state brain [¹⁸F]fluorodeoxyglucose positron emission tomography (PET) scanning twice: first at the baseline and then at the 5-year follow-up. We analyzed the effect of day length on brain glucose uptake separately for both time points. In both baseline and follow-up scans, day length on the day of imaging significantly predicted glucose uptake in the socio-emotional circuit. A longer day length was associated with increased glucose uptake in the cuneus, precuneus, orbitofrontal cortex, pre- and postcentral gyrus, superior and middle temporal gyrus, posterior cingulate cortex, insula, and frontal pole. This large-scale longitudinal PET study provides landmark evidence for the impact of daylight exposure on brain glucose metabolism. Findings disclose the baseline seasonal variation of brain energy consumption in men.