Enhancement of paradoxical sleep by lights-off stimulation depends on sleep states

This study was aimed at clarifying whether the lights-off stimulation effect on paradoxical sleep (PS) enhancement in albino rats depends on sleep and wakefulness states. Male Sprague-Dawley rat pairs were subjected to lights-off stimulation for 320 s when one of the two rats was in non-rapid-eye movement sleep (NREM) continuously for at least 64 s (NREM-off condition). Simultaneously, the other rat received the same lights-off stimulation during any sleep and wakefulness state (random-off condition). Each rat underwent both lights-off conditions. The sequence of the two lights-off conditions was counter-balanced. PS was enhanced during the lights-off period in both the NREM-off and random-off conditions. However, the increased amount of PS was greater in the NREM-off condition than the random-off condition. In the random-off condition, rats had more PS in the lights-off period than in the preceding and succeeding lights-on periods only if they were in high EEG amplitude NREM (HS) at lights-off. Thus, this study verified that the lights-off stimulation effect on PS enhancement in the albino rat depended on the sleep states. However, the lights-off treatment did not alter the relationship between PS and preceding NREM, or the hippocampal theta activity or the PS latency in the lights-off period.