Human energy expenditure (EE) is not stable over a 24 hour period, it has its own rhythm. It varies depending on the time of the day, the physical activity level and the exercise performed by the subject, the amount of food ingested and its composition. The human 24-hour EE is made up of several components. Each component contributes to an individualized response and rhythm, therefore the different components can be identified as different stages of the 24h EE. By assigning different components to a specific time period, it is possible to measure the different components of the 24-hour EE with indirect calorimetry (IC) equipment. In this web series, we will discuss and explain all EE components one by one. We start the series with the SMR, i.e. the sleeping metabolic rate.

The SMR does not have a fixed or stand-alone definition, because there are different views among studies and researchers on the norm of the definition. One suggestion is to calculate the SMR at the lowest EE measured over the night and over three adjacent hours. The alternative approach is that the SMR is determined by the energy expenditure during the time period when the amount of activity is lowest, usually over a period of three consecutive hours. However, when using the latter approach, equipment capable of measuring human movement needs to be implemented. These can be accelerometers or indoor motion sensor systems. Although SMR is sometimes determined by using a metabolic cart, the use of a whole room calorimeter (WRC) is much more common. These WRC systems are much more advanced and incorporate all the technical features and instrumentation to measure the subject under the most optimal conditions. Consider the free-living component and climate control.


When measuring the SMR earlier in the night, the thermal effect of food (TEF) will continue to have an effect for at least four to six hours after the last meal.  This could limit the measurement, as the EE decreases during sleep. For the assessment of the SMR, this would mean that it varies from the early night (the EE would be higher) to the late night (the EE would be lower). This gradual decrease during the night is thus partly explained by the TEF component at the beginning of sleep. An SMR measurement has another limitation: the “first night effect”. This can lead to more periods of wakefulness. This can be easily solved by measuring at least two nights in a row, since after one night the sleep disturbances disappear. On the other hand, an SMR measurement is a highly reproducible 24-hour EE component, with the lowest intra-subject day-to-day coefficient of variation. This offers great opportunities to determine metabolic effects within subjects very accurately.

More information and the recommendations for performing an indirect calorimetry measurement can be found in the RICORS 1.0 (Room Indirect Calorimetry Operating and Reporting Standards). An article written and compiled by the specialists in the field.

If you are interested in one of our indirect calorimetry cart (hood and basic room), whole room calorimeter systems or accelerometry systems, please contact us or find more information on our information pages.



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  • Alcántara Alcántara, Juan Manuel. Assessment of resting energy expenditure and nutrient oxidation by indirect calorimetry: methodological implications. Granada: Universidad de Granada, 2021.


  • Spaeth, Andrea Marie, “Consequences of Chronic Sleep Restriction on Energy Balance in Healthy Adults” (2014)