Metabolic rate is one of the main parameters affecting the thermal balance of the human body and perception of thermal comfort. Typically, we consider a constant value for a specific type of activity (sitting quiet, standing, etc.) despite the time of the day and body built of a person. In this study, we challenged this by undertaking minutely metabolic rate measurements of 3 men in a realistic office layout at 23-27°C operative temperature. The type of activity (sitting and standing quiet work) and their timing were standardized, and the meal ingested (breakfast and lunch) by the participants was also the same. We also measured temporal changes in the heart rate and skin temperature to understand the variation of physiological parameters. A whole day session was split into four 1.5-hour-long sessions, two in the morning (08:30-12:00) and two in the afternoon (13:00-16:30). Thermal comfort between sessions varied within ±1 per ASHRAE seven-point thermal sensation scale, based on the surveys. The metabolic rate varied throughout the day, even for the same activity type, with an apparent effect of the activity performed upon arrival in the morning (e.g., commute to the office) and the thermic effect of food. After a standardized normal-protein lunch, the metabolic rate was about 15% higher for the same activity for all three men. The effect of the prior physical activity on the metabolic rate was smaller than the meal effect. These results revealed that people’s metabolic rate is dynamic, and it can be elevated not only because of physical activity but also by diet-induced thermogenesis. All in all, this work is intended to draw attention to the metabolic rate variation in daily life that has been overlooked so far in the field of ergonomics of the indoor environment and to outline possible future perspectives for smart buildings if personalized metabolism could be known.