Regional brain glucose metabolism during morning and evening wakefulness in humans: Preliminary findings

Study Objectives: The mechanisms that maintain wakefulness across the day, in the face in increasing sleep drive, are largely unknown. The goal of this pilot study was to examine regional relative brain glucose metabolism during morning and evening wakefulness in healthy humans. Design: [¹⁸F]- fluorodeoxyglucose positron emission tomography scans were conducted during quiet wakefulness in the morning and in the evening. Statistical parametric mapping was used to compare relative regional glucose metabolism during the 2 scans. Subjects also completed subjective ratings of alertness. Setting: University of Pittsburgh General Clinical Research Center and Positron Emission Tomography Facility. Patients or Participants: Thirteen healthy adults (10 women, 3 men; mean age, 37 years) Interventions: None. Measurements and Results: Relative regional glucose metabolism was significantly higher in the evening than in the morning in a large cluster of midline and brainstem structures. Volumes of interest centered on the pontine reticular formation, midbrain reticular formation, midbrain raphe, locus coeruleus, and posterior hypothalamus also showed higher relative metabolism in the evening than in the morning. Relative glucose metabolism was significantly lower in the evening than in the morning in clusters that included structures in the right temporal cortex and occipital lobe, including cuneus and medial occipital gyrus. Conclusions: Evening wakefulness is associated with increased relative metabolism in brainstem and hypothalamic arousal systems and decreased relative metabolism in posterior cortical regions. These patterns may reflect input from the circadian timing system to promote wakefulness, and/or the effects of increasing homeostatic sleep drive.