The effect of time of day on apnoea index in the sleeping rat.

This study tested the hypothesis that apnoea index would be greater during daytime sleep than nighttime sleep in the rat. Electroencephalogram and electromyogram were monitored via biotelemetry implant and respiration was measured using whole body plethysmography in six male rats in two separate 34h recording sessions per animal. Apnoeas were classified as "spontaneous" or "post-sigh". Daily average spontaneous apnoea index was 35 times greater (p<0.0001) during rapid eye movement (REM) sleep than in non-REM (NREM) sleep. In contrast, daily average post-sigh apnoea index was not significantly greater in REM sleep than in non-REM (NREM) sleep (p=0.39). There was a greater post-sigh apnoea index during daytime REM than during nighttime REM (p=0.043) but REM-related spontaneous apnoea index was unaffected by time of day. There was no day to night difference in spontaneous apnoea index or post-sigh apnoea index during NREM sleep. Respiratory variability (coefficient of variation for breath duration and tidal volume) was not affected by time of day in REM or NREM sleep. We conclude that the circadian timing system has no effect on apnoea index during NREM sleep in the rat, but it may influence the propensity for post-sigh apnoea during REM sleep.