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Circadian rhythms and sleep have additive effects on respiration in the rat

Richard Stephenson *†, Kiong Sen Liao *, Hedieh Hamrahi †‡ and Richard L. Horner *‡

1. We tested two hypotheses: that respiration and metabolism are subject to circadian modulation in wakefulness, non-rapid-eye-movement (NREM) sleep and rapid-eye-movement (REM) sleep; and that the effects of sleep on breathing vary as a function of time of day.
2. Electroencephalogram (EEG), neck electromyogram (EMG) and abdominal body temperature (T_b) were measured by telemetry in six male Sprague-Dawley rats. The EEG and EMG were used to identify sleep-wake states. Ventilation (_I) and metabolic rate (_CO2) were measured by plethysmography. Recordings were made over 24 h (12:12 h light:dark) when rats were in established states of wakefulness, NREM sleep and REM sleep.
3. Statistically significant circadian rhythms were observed in _I and _CO2 in each of the wakefulness, NREM sleep and REM sleep states. Amplitudes and phases of the circadian rhythms were similar across sleep-wake states.
4. The circadian rhythm in _I was mediated by a circadian rhythm in respiratory frequency (f_R). Tidal volume (V_T) was unaffected by time of day in all three sleep-wake states.
5. The 24 h mean _I was significantly greater during wakefulness (363.5 ± 18.5 ml min^-1) than during NREM sleep (284.8 ± 11.1 ml min^-1) and REM sleep (276.1 ± 13.9 ml min^-1). _CO2 and V_T each significantly decreased from wakefulness to NREM sleep to REM sleep. f_R was significantly lower in NREM sleep than in wakefulness and REM sleep.
6. These data confirm that ventilation and metabolism exhibit circadian rhythms during wakefulness, and NREM and REM sleep, and refute the hypothesis that state-related effects on breathing vary as a function of time of day. We conclude that the effects of circadian rhythms and sleep-wake state on respiration and metabolic rate are additive in the rat.

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