Interaction between light-dark cycles and circadian rhythm on sleep and wakefulness in albino rats

This study investigated the role of the circadian phase in modulating the effect of short light-dark cycles (LDc) on sleep and wakefulness. Six male albino rats of the Sprague-Dawley strain were implanted with electrodes for standard electrophysiological recordings performed during baseline (12 - 12 h LDc), short LDc treatment, and recovery (12 - 12 h LDc) for 4 days each. In the short LDc treatment, 15 - 15 min LDc were applied, respectively, in mid-periods of inactive and active phases to maintain an entrained circadian rhythm. The results showed that the 15 - 15 min LD ratio of both non-rapid eye movement sleep (NREM) and paradoxical sleep (PS) did not vary with the circadian phase. In contrast, changes in both the NREM and PS amounts in the short LDc treatment varied with the circadian phase. It is argued in the Discussion section that the circadian phase-related changes in the sleep amount did not result from the circadian rhythm effect but from the interactions between the habitual 24 h lighting schedule and the habitual LD distribution of the sleep and wakefulness amounts. On the other hand, this study found that both waking (W) and PS response to short LDc varied with time courses. The 15 min dark period strongly enhanced the W time only when it occurred for the first time in the inactive phase while it consistently facilitated PS across the remaining time periods in both the active and inactive phases. Furthermore, a residual effect of short LDc on PS was revealed in this study. Compared to the baseline, the 12 - 12 h LD ratio of PS was significantly decreased during recovery compared to the short LDc treatment.