Implementing heating and cooling set-point temperature modulations in buildings can promote energy savings and boost energy flexibility. However, time and time-of-day requirements in current indoor climate regulations are either overly simplified or ignored completely. A better understanding of how human thermal responses vary throughout the day is useful to effectively design and operate energy-flexible buildings. To date, only a handful of studies have looked at diurnal changes in thermal perception and mostly near steady-state neutrality without controlling for light exposure. This is the first experimental investigation aimed at understanding how the time of the day influences physiological and subjective human sensory responses to a localized dynamic thermal stimulus under constant light rich in long wavelengths (red). Results indicated that humans responded physiologically differently depending on the time of the day with a higher rate of change in the skin temperature in the evening compared with the afternoon. Furthermore, the increase of thermal sensation during the warming skin temperature transients was found to be greater in the evening. No differences were observed under steady-state thermal conditions. This evidence suggests that accounting for the time of the day is important when dynamically operating buildings, such as during demand-response programs.
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