Heatstroke is caused by overheating, typically as a result of prolonged physical exertion at high temperatures. As the most serious type of heat injury, heatstroke occurs if the body temperature rises to 40 °C. Heatstroke often occurs when there is a sudden change in ambient temperature (e.g., from spring to summer) and the thermoregulation system is not functioning optimally. The risk of heatstroke at high temperatures differs according to age. Heatstroke requires emergency treatment. Untreated heatstroke can cause permanent damage to the brain, heart, kidneys, and muscles. Continuous monitoring of vital signs can provide physiological information, and a digital marker sensitive to subtle physiological changes may be useful for disease prevention. A shift in vital-sign measurement from a hospital to an ambulatory setting has been facilitated by the popularity of wearable devices.