Prep = subject preparation, including placement of sound-attenuating ear plugs, adhesive electrodes, and foam head restraints. Localizer = anatomical localization scan to position the imaging volume. T1 = T1-weighted anatomical scan. T2* = 15 min BOLD image-volume acquisition for ICN analysis. ASL = 2.5 min arterial spin labeled acquisition. The number of epochs of T2* (1-3) and ASL data (3-5) varied across subjects, depending on sleep duration. SS = sleep-stage determination, which was performed between all imaging epochs.