The purpose of this study was to examine metabolic rate and substrate oxidation during sleep in relation to time of sleep and sleep stage. Twelve male subjects free from sleep-disordered breathing slept for 469 +/- 8.7 (mean +/- SE) minutes until natural awakening in a whole-body indirect calorimeter, and polysomnographic documentation of sleep was recorded. Energy expenditure decreased during the first half of the night, reached a nadir (a 35% decrease), and remained relatively stable until awakening. Similarly, fat oxidation decreased from the onset of sleep. On the other hand, carbohydrate oxidation showed no remarkable changes from the onset of sleep but began to increase before awakening. Because distribution of sleep stages is not uniform throughout the night, with rapid-eye-movement (REM) sleep tending to appear later in the sleep, effect of sleep stage on energy metabolism was isolated by analysis of covariance with time as a covariate. Subsequent comparison of metabolic rate by 1-way analysis of variance with Bonferroni post hoc analysis revealed that energy expenditure during REM sleep was significantly greater than that during sleep stages 2 and 3/4 (stage 2, 25.248 +/- 0.961; stage 3/4, 24.825 +/- 0.935; REM, 25.712 +/- 0.928 kcal kg(-1) fat-free mass d(-1)). Carbohydrate oxidation during REM sleep was significantly greater than that during sleep stage 3/4 (stage 3/4, 12.229 +/- 1.071; REM, 13.986 +/- 1.291 kcal kg(-1) fat-free mass d(-1)). Respiration quotient was statistically different among sleep stages, but Bonferroni post hoc analysis failed to identify significant differences (stage 2, 0.850 +/- 0.010; stage 3/4, 0.846 +/- 0.011; REM, 0.861 +/- 0.013). The increases in energy expenditure and carbohydrate oxidation during REM sleep are consistent with a notion that changes in energy metabolism in brain are manifested as small fluctuations in whole-body energy metabolism during sleep.