Effects of intermittent cooling on human thermophysiological and perceptual responses in a non-steady-state thermal environment

Intermittent cooling, characterized by the on-off cycling of air-conditioning systems, generates a non-steady-state indoor thermal environment. While widely adopted for energy conservation, its impact on human thermophysiological and perceptual dynamics remains insufficiently understood. To investigate this, we conducted chamber experiments in a simulated residential/office space using three cooling set-points (thermostat target temperature: 28 °C, 26 °C, and 20 °C). Each trial involved 70 min of active cooling followed by 50 min of shutdown. We continuously monitored indoor air temperature, skin temperature, blood pressure, heart rate, and subjective thermal responses from 12 participants throughout each cycle. Results show that cold exposure intensity significantly shaped the temporal dynamics of human thermal response. Under strong cooling (20 °C), skin temperature declined continuously without stabilization over 70 minutes, thermal sensation and acceptability remained unstable until 35 - 40 minutes, and cardiovascular responses (e.g., DBP, HR) were only activated after prolonged exposure, stabilizing after > 50 minutes. In contrast, moderate and weak cooling (26 - 28 °C) enabled most physiological and perceptual responses to stabilize within 15 - 25 minutes. Asynchronous was observed among systems: skin temperature stabilized earliest, followed by perceptual responses, while cardiovascular regulation remained notably delayed under stronger cooling. After AC shutdown, overall recovery occurred within 35 minutes transitional time-frame, during which the effects of residual cold exposure continued to influence thermal perception and physiology. Based on these findings, we propose a theoretical framework for a “Cumulative Thermal Stimulus (CTSI)” index that integrates thermal deviation and exposure duration, offering a quantitative tool for adaptive cooling strategies that optimize comfort, health, and energy efficiency.