Fig. 7: 24-Hour Time Series of HR, CBT, and PSI by Sex.

Panel (a) shows the relative change in heart rate (HR) over the course of the day, calculated as the percentage increase relative to the individual’s heart rate reserve (HRR). Panel (b) shows the relative change in core body temperature (CBT), calculated as the percentage increase relative to the individual’s CBT reserve capacity. Panel (c) displays the Physiological Strain Index (PSI), a composite measure of heat strain, which reflects the combined impact on heart rate and CBT. In all panels, females (blue) and males (red) are compared across different times of day.