Oxygen consumption during sleep: influence of sleep stage and time of night

We measured oxygen consumption (VO2) in eight normal male volunteers during sleep, using the ventilated-hood method. Data were collected over 28 subject-nights. There was an overnight trend of gradually decreasing VO2 in the first 4 h, followed by a rise toward the morning. The minimum VO2 was 7.9% lower than that in the first hour. To examine the influence of sleep stages on the VO2, we compared the VO2 of a sleep stage (an overnight average of all epochs in that stage) with that of other stages. The results show that VO2 values in stages awake and 1 are significantly higher than all other stages. Stage rapid eye movement (REM) is significantly lower than stage 2, but stages 3 and 4 are not different from each other or from stages REM and 2. We also compared VO2 of sleep stages that occurred close to each other (within the same hour). VO2 in awake stage is again significantly higher than in all other stages, and stage 2 is higher than stages 3 and 4. However, no difference is found between stage 1 and stages 2, 3 and REM, nor is there any difference between REM and stages 2 and 3. The discrepancy between close-stage comparison and overnight-average comparison can be accounted for by the variation in VO2 of an individual stage with the time of night. Although there is a variation in time distribution of the stages overnight, this factor influences the overnight trend of VO2 in a minor fashion only.