NREM and Wake EEG delta power. Delta power (mean % of baseline ± SEM) was determined across 18-h (ZT6-24, open bar) and 6-h (ZT0-6, gray bar) time blocks on baseline (BL), sleep restriction (SR1-SR5) and recovery sleep (R1-R3) days. NREM delta power (top panel) was increased significantly after the first 18-h of sleep deprivation (SR1), but not on SR2 through SR5. For ∼ 1 h of sleep obtained during the 18-h sleep deprivation (SD) periods, NREM delta power did not increase, but actually decreased significantly on SR3 to SR5 (all P < 0.05). The wake delta power (bottom panel) was not significantly increased over 9 experimental days, suggesting the absence of dissipating delta power into the rats' wake state. The asterisk (*) indicates statistical significance (P < 0.05, N = 8) compared to the corresponding baseline.