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Footbath has been a common form of activity in China, Egypt and India for several hundred years. The beneficial effect of footbaths on sleep has been already known. However, the optimal footbath conditions for improving sleep quality have received little attention. To investigate the effect of water temperature and footbath duration on body temperature responses during and after footbath using a thermoregulatory modelling approach. A classical multi-node human thermoregulation model was modified to account for human exposure to warm water locally while seated during a footbath. The model was then used to systematically examine the effects of water temperature and footbath duration on body temperature responses during footbaths as well as to estimate the most optimal footbath duration in different water temperatures to improve sleep. This thermoregulatory modelling study revealed that the mean skin temperature after a footbath was higher than before, whereas the core temperature after footbath was lower. The higher the water temperature and duration of the footbath, the longer time that the core temperature was maintained with a decreasing trend while the core temperature was higher. The mean skin temperature would no longer rise if there was sweating during the footbath. The onset of sweating was roughly inversely proportional to the temperature of the water. The discrete thresholds thermoregulation model was able to predict body temperature responses during a footbath. A relationship curve between sweating onset time and water temperature has been established to determine the optimal conditions of footbath, which may provide reference to facilitate sleep onset. [Display omitted] • A classical thermoregulatory model was extended to predict body temperature during footbath. • Footbath increased mean T skin , DPG and decreased T core thus improved sleep. • Optimal duration of footbath at 42 °C was 24 min when sweating just occurs. • Heat can be transferred from the feet to the whole body effectively during footbaths. • The optimal footbath duration is determined by the sweating onset time. [ABSTRACT FROM AUTHOR]