Recommendations for daily energy requirements use factorial calculations based on BMR. Expressing total energy requirements as a multiple of BMR is based on the assumption that BMR is equal to overnight metabolic rate (OMR). The objective of the present study was to determine if BMR is an appropriate proxy for OMR in children, young adults and elderly. Data are presented of thirty children (11 (sd 2) years), thirty young adults (25 (sd 5) years) and fifty-nine elderly(61 (sd 5) years). OMR was measured in a respiration chamber while sleep was not hindered and BMR was measured directly afterwards with a ventilated hood system under strictly controlled conditions. The mean ratio of OMR:BMR was 0·92 (sd 0·09) for children, which was significantly different from 1·00 (P<0·001), 1·00 (sd 0·07) for young adults and 1·06 (sd 0·09) for elderly whichwas also different from 1·00 (P<0·001). Foradults, BMR is an appropriate measure of OMR. In children, the use of BMR to estimate OMR wouldintroduce an overestimate and for elderly an underestimate.