Figure 2.

Heart rate variability (HRV) measures obtained during each vigilance state (i.e., Wake: wake episodes; WBSO, wake before sleep onset; WASO, wake after sleep onset; S1, stage 1 sleep; S2, stage 2 sleep; SWS, slow wave sleep; REMS, rapid eye movement sleep). HRV measures were binned according their associated vigilance state, regardless of the nap number or circadian phase. The Y axis of HRV measures represents the % deviation from the subject's mean across the ultradian sleep-wake cycle procedure. Thus, a value of 0% represents individuals' mean. For each HRV component, letters above each column are used to indicate significant differences between vigilance/sleep stages, i.e., when two vigilance/sleep stages have no common letter, there are significantly different (P < 0.05). All values are mean ± standard error of the mean.