Context: Many studies involving measurement of energy expenditure are often confused with the term "basal metabolic rate" (BMR) and “resting metabolic rate” (RMR). RMR is usually carried out using a canopy, over a short time (less than one hour), with subjects in decubitus position, quiet but awake. However, the lowest values of energy metabolism (BMR) can only be measured when the volunteer is deeply asleep, i.e. during the night. Methodological tools such as calorimetric chambers are able to perform both measurements since they continuously measure all kinetics changes of energy expenditure over 24 hours or more. Methods: Among several studies regarding energy expenditure using two calorimetric chambers, BMR were collected from the sleep periods by considering at least 2 consecutive hours for which the volunteers were asleep, when their heart rate and energy expenditure were low. Then, volunteers were also asked to lie down on their bed in the morning just after waking, such like under a canopy, for RMR measurement. Results: Overall, 48 men were included with mean age 57±1.2 years. Total EE of the individuals was 2326±47 kcal/24h. BMR was significantly different from RMR (62.2±1.3 and 75.5±1.7 kcal/h respectively, P<0.0001), so that it represents 64.2 and 77.9 % % of total EE, respectively. In addition, positive correlations were found between BMR, RMR and body weight: BMR and RMR were correlated to fat free mass (0.346 vs 0.546, P<0.05). Conclusion: Energy expenditure during BMR is 17.6 % lower than RMR. The investigations often referring to the basal metabolism rate should rather talk about the resting metabolism rate when they are not asleep. The basal metabolism rate can only be measured during sleep and calorimetric chambers are appropriate for this activity