Thermoregulation during mild exercise at different circadian times

Area of Science:

• Human physiology
• Chronobiology
• Exercise science

Background:

• Core body temperature exhibits a significant circadian rhythm.
• Thermoregulatory responses can be influenced by the timing of physical activity within this rhythm.

Purpose of the Study:

• To investigate how the timing of exercise affects core temperature regulation and physiological responses.
• To determine if the body's ability to dissipate heat during exercise is time-dependent.

Main Methods:

• Healthy subjects performed standardized exercise at different times of day, synchronized with their core temperature rhythm.
• Measurements included core temperature, skin blood flow, heart rate, perceived exertion, and sweat loss.
• Core temperature changes were modeled using a negative-exponential function.

Main Results:

• Exercise significantly reduced the amplitude of the core temperature circadian rhythm.
• The rate and extent of core temperature rise during exercise were inversely related to resting temperature.
• Thermoregulatory responses, including sweat loss and skin blood flow, were generally greater in the evening compared to the morning.
• Exercise kinetics did not appear to be time-of-day dependent based on the applied model.

Conclusions:

• The body's capacity to manage heat load during exercise is influenced by the circadian rhythm of core temperature.
• Physiological responses to exercise, particularly heat dissipation, show significant diurnal variations.
• These findings have implications for understanding human performance and adaptation to environmental heat stress across the day.