Background: The core interthreshold zone (CIZ) is defined as the range between temperatures at the onset of shivering and sweating. Its circadian or diurnal variation has not been extensively studied. The present study examined whether the CIZ is subject to a diurnal rhythm. In addition, according to the previous finding that the CIZ was proportionally correlated with peripheral interthreshold zone (PIZ), it was also examined whether cutaneous sensation threshold zone (CSZ), a determinant of the PIZ, is correlated with the CIZ. Methods: The CIZ and the CSZ were measured in ten Japanese men who underwent three experiments in a single day on the morning, afternoon, and evening in the 2014 experiment (so-called single-day experiment) and six Japanese men underwent the same experiments on the morning of day 1, the afternoon of day 2, and the evening of day 3 in the 2015 experiment (so-called multiple-day experiment). Air temperature was controlled at 20-24 °C. Each subject wore a suit perfused with 25 °C water at a rate of 600 cm3/min and exercised on an ergometer at 50% of their maximum work rate for 10-15 min until their rate of sweating increased. They then remained seated without exercising until their oxygen uptake increased. Rectal temperature, skin temperatures at seven sites, the sweating rate at the forehead, and oxygen uptake were continuously monitored throughout experiment. Cutaneous warm and cold sensation thresholds at three sites were measured using 1- and 2-cm2 probes. Results: The results from the single-day experiment demonstrated a small change in the CIZ and core temperature prior to exercise (Tc-init) whereas those from the multiple-day experiment demonstrated continuous increase in the CIZ and Tc-init. The CSZ measured with a 1-cm2 probe was inversely proportional to the average skin temperature at three sites prior to measurement (Tsk-av). Conclusion: The results suggested that the CIZ may be not dependent on time of a day but Tc-init per se and may not be associated with the CSZ. [ABSTRACT FROM AUTHOR]