The first metabolic chamber for human subjects in Japan was built in 2000 in National Institute of Health and Nutrition.In 2019, 19 metabolic chambers, including 4 chambers in the University of Tsukuba, have been built in Japan. Now, China is catching up with us importing metabolic chambers from Japan, which seems to approve our decision back in 2003 to improve time resolution of the system. This is a personal account on the history of metabolic chamber, which is also known as a whole room indirect calorimeter or human calorimeter, during the recent 20 years. Funded by COE program, the second metabolic chamber in Japan was built in University of Tsukuba in 2003. In our system, an online mass spectrometer was installed and a new algorithm was developed to improve transient response of metabolic chamber. Currently, time resolution of our system is the best in the world. Using our system, it was shown that 1) effect of exercise to increase accumulated fat oxidation during 24 h depends on when the exercise is performed, 2) energy metabolism during sleep differs among sleep stages, 3) energy expenditure and RQ decrease during the first half of sleep followed by a gradual increase prior to awakeni
