Figure 4. Effect of chronic sleep restriction on sleep homeostasis. A, Schematic showing the protocol for sleep deprivation (SD) after sleep restriction: mice were housed on a treadmill (TM) and monitored with cortical EEG. In the nonsleep-restricted (Non-SR) group, mice were left undisturbed on the TM while in the sleep-restricted (SR) group, mice were subjected to 3 d (72 h) of restricted sleep. Assessment of sleep homeostasis was successively conducted on days 6 and 19 of the experiment for the non-SR group and after 24 h and 2 weeks of recovery for the SR group. Both non-SR and SR mice were sleep deprived for 6 h on the TM starting at ZT0 and then allowed to recover for the following 18 h. B–E, Assessment of sleep homeostasis on day 6. B, Following SD, the increase in SWA power during NREM sleep, expressed as percentage of last 4 h of the light phase during day 5 was significantly smaller in the SR animals compared with the non-SR animals (two-way repeated-measures ANOVA: condition, F = 13.50, p < 0.001; time, F = 41.45, p < 0.001; condition × time, F = 8.02, p < 0.001; SNK post hoc test, *p < 0.05, **p < 0.01, ***p < 0.001; n = 16 animals per condition). C, Average latency to NREM sleep after SD in non-SR and SR animals (Mann–Whitney U test: U = 98.50, p > 0.05, n = 16 animals per condition; n.s., nonsignificant). D–E, SD increased both the amount of time spent in NREM sleep (D) and duration of NREM episodes (E) in non-SR group but not in SR group during the 18 h recovery period compared with the corresponding 18 h of day 5 (D: non-SR group, paired Student's t test, t = −4.58, ***p < 0.001; SR group, paired Student's t test, t = 0.80, p > 0.05, n = 16 mice per group; E: non-SR group, paired Student's t test, t = −4.29, ***p < 0.001; SR group, paired Student's t test, t = −0.56, p > 0.05, n = 16 animals per group; n.s., nonsignificant). F–I, Assessment of sleep homeostasis on day 19. F, Following SD, the increase in SWA power during NREM sleep, expressed as percentage of last 4 h of the light phase during day 18 was significantly smaller in the SR animals compared with the non-SR animals (two-way repeated-measures ANOVA: condition, F = 1.34, p > 0.05; time, F = 16.78, p < 0.001; condition × time, F = 2.46, p < 0.05; SNK post hoc test, **p < 0.01; n = 6 animals per condition). G, Average latency to NREM sleep after SD in non-SR and SR animals (unpaired Student's t test: t = −0.91, p > 0.05, n = 6 animals per condition). H–I, SD increased both the amount of time spent in NREM sleep (H) and duration of NREM episodes (I) in non-SR group but not in SR group during the 18 h recovery period compared with the corresponding 18 h of day 18 (H: non-SR group, paired Student's t test, t = −2.86, *p < 0.05; SR group, paired Student's t test, t = −1.55, p > 0.05, n = 6 mice per group; I: non-SR group, paired Student's t test, t = −3.33, *p < 0.05; SR group, paired Student's t test, t = −1.60, p > 0.05, n = 6 animals per group). Error bars represent SEM.