RELATIONSHIP BETWEEN SLEEP STAGES AND METABOLIC-RATE IN HUMANS

Differences in sleeping metabolic rate (SMR) among subjects may be rel ated to-different levels of energy expenditure associated with sleep s tages. The relationship between energy expenditure and sleep stages wa s investigated overnight in 29 subjects (14 Caucasians and 15 Pima Ind ians, 18 males and 11 females; mean +/- SD, 31 +/- 7 yr, 83 +/- 26 kg, 27 +/- 11% fat). Sleep stages were determined by electroencephalogram recording, whereas energy expenditure was measured in a 1,000-liter P lexiglas sleep box constructed around a bed as a fast-response open-ci rcuit indirect calorimeter. Eighty-five percent of the interindividual variability in SMR was explained by differences in fat-free mass, fat mass, age, sex, and race (r(2) = 0.85). The intra-individual variance in SMR over time was related to sleep stages and to clock time. Withi n subjects, SMR in stage 3 was significantly lower than in stage 2 (-3 9 +/- 18 kcal/day; P < 0.05) and rapid eye movement sleep (-51 +/- 23 kcal/day; P < 0.05). Also, sleep stages were associated with different respiratory quotients. Because sleep stages are associated with only small differences in energy metabolism, our results suggest that sleep stages play a minor role in the variance of SMR among subjects. Howev er, the duration of sleep may contribute to the variability of 24-h en ergy expenditure.