Voxel-based morphometric gray matter correlates of daytime sleepiness

► Structural magnetic resonance imaging data were acquired in healthy men and women. ► Participants provided ratings of average daytime sleepiness in various settings. ► We correlated daytime sleepiness with medial orbitofrontal cortex volume. ► Smaller orbitofrontal cortex volume correlated with greater sleepiness ratings. ► Medial orbitofrontal cortex volume may play an important role in sleepiness.