Subcortical gray matter volume by time of day. A, Multivariate regression for subcortical regions demonstrated decreasing volume throughout the day. Left, Purple tones indicate volumetric reduction while aqua tones indicate volumetric expansion from A.M. to P.M. (FDR at q < 0.05; nonsignificant test statistics were set to zero for interpretability). Gray matter volume was higher in the right cerebellum, brainstem, and right hippocampus in morning compared with evening sessions (see example box plots on the right). For the full list of subcortical results, see Extended Data Table 4-1. B, Diagram illustrates medial temporal lobe subregion segmentation from the ASHS parcellation. Gray matter volume in CA1, dentate gyrus, and subiculum did not change throughout the day. This pattern held for all medial temporal lobe subregions (CA2/3, entorhinal cortex, perirhinal cortex, and parahippocampal cortex, p > 0.007 for all, Extended Data Table 4-2). For a manual segmentation example, see Extended Data Figure 4-3. Abbreviations: GMV, gray matter volume; ns, not significant.