Fig. 2. Group differences (analysis of covariance) for power across successive sleep periods in (A) HF band and (B) LF:HF ratio. Mean values are adjusted for ambient stress at baseline and wakefulness during sleep. HF power is represented as milliseconds squared/hertz, and LF:HF ratio is represented as the ratio of LF to HF spectral power. Error bars represent SD. *Group by time interaction is significant at the p <.05 level.