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

1. We tested two hypotheses: that respiration and metabolism are subject to circadian modulation in wakefulness, non-rapid-eye-movement, (NREM) steep and rapid-eye-move ment (REM) sleep; and that the effects of steep 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 steep-wake states. Ventilation ((V) over dot(1)) and metabolic rate ((V) over dot(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 steep and REM steep. 3. Statistically significant circadian rhythms were observed in (V) over dot(1) (V) over dot(CO2), and in each of the wakefulness, NREM steep and REM steep states. Amplitudes and phases of the circadian rhythms were similar across sleep-wake states. 4. The circadian rhythm in (V) over dot(1) was mediated hy 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 (V) over dot(1) was significantly greater during wakefulness (363.5 +/- 18.5 ml min(-1)) than during NREM steep (284.8 +/- 11.1 ml min(-1)) and REM steep (276.1 +/- 13.9 nil min(-1)). (V) over dot(CO2) and V-T each significantly decreased from wakefulness to NREM sleep to REM steep. f(R) was significantly lower in NREM steep than in wakefulness and REM steep. 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 oil respiration and metabolic rate are additive in the rat.