Fig. 4. The percentage of time during the 12-h dark phase that rats (n = 7) spent in states of wakefulness (Wake), nonrapid eye movement sleep (NREM), and rapid eye movement sleep (REM). These recordings were obtained during hours 12 through 24 after administration of either saline (control) or buprenorphine. Asterisks (*) indicate significant differences compared with saline. These results illustrate that after the buprenorphine-induced inhibition of sleep during the light phase (Fig. 3A) there was a significant rebound increase in NREM sleep and REM sleep during the dark phase, when these rats normally would be awake.