¹⁸F-FDG Brain PET/CT is widely used to assess cerebral glucose metabolism. The occipital cortex is particularly sensitive to visual stimulation and may demonstrate altered metabolic activity depending on uptake conditions. Current guidelines recommend that patients remain awake with eyes open in a dark environment during the uptake phase; however, environmental control measures are not clearly standardized. A man in his early 30s underwent ¹⁸F-FDG Brain PET/CT. The initial scan demonstrated marked hypermetabolism in the occipital cortex. Reassessment of the uptake conditions revealed unintended exposure to external light despite adherence to instructions. A repeat scan was performed with the patient wearing an eye mask during uptake. On the repeat scan, occipital hypermetabolism resolved, and no significant hypometabolic abnormality was observed. This case illustrates that even minimal visual stimulation can significantly influence occipital FDG uptake and alter relative metabolic distribution. Such variability may confound image interpretation, particularly in neurodegenerative disorders such as dementia with Lewy bodies, in which occipital hypometabolism is characteristic. Strict environmental standardization, clear instructions, and documentation of eye status may improve diagnostic reliability. When environmental control is difficult, the use of an eye mask may be considered.