Oxygen consumption, circadian rhythmicity and sleep

Oxygen consumption is lower during sleep than relaxed wakefulness. However, there is disagreement as to the particular metabolic changes which produce the difference. The present study assessed (i) the contribution of sleep, circadian cycle and the specific dynamic action effect of the evening meal, to the fall in metabolic rate during the sleep period. (ii) the effects of sleep stage on oxygen consumption which had been suggested by previous researchers, and (iii) the effect of body movement arousals on oxygen consumption. Five subjects were tested for a total of nine nights under three conditions in a repeated measures design. Subjects were confined to bed throughout their usual sleep period, but were allowed to go to sleep 0, 3 or 6 hours following their usual time for lights out. Oxygen consumption was measured in all conditions for the half hour before and after each of the times for lights out and then throughout the sleep period following lights out. The results demonstrated that changes in energy expenditure during the sleep period are a function of both sleep and circadian cycle. In this study the contribution of the components was approximately equal. However, the effect of sleep was rapid, with oxygen consumption values reaching an asymptote within fifteeen minutes of sleep onset, while the effect of circadian cycle was constant over the assessment period. No evidence was found implicating the specific dynamic action effect of the evening meal in the reduction in sleep period metabolic rate. The results of previous studies can be interpreted as being due to the combined effect of circadian and sleep influences, and not to specific differences in metabolic rate between separate sleep stages. These would appear to be artifactual. In addition, the period of metabolic disturbance following a movement arousal was shown to be longer than that suggested by previous researchers.