This study investigates the impact of circadian rhythms on thermal adaptation during a four-day controlled exposure to moderate heat at different times of the day (morning vs. midday) and under varying lighting conditions (dim vs. bright). The findings suggest that warmth adaptation is more tolerable in the morning than at midday under equivalent thermal conditions. This effect is supported by a decrease in mean skin temperature and a perceptual shift toward more neutral thermal sensations over the four days. The lower mean skin temperatures observed in the morning indicate that the circadian rhythm of skin temperature may enhance heat tolerance, leading to less strenuous adaptation to warm environments. These results highlight a potential circadian influence on thermal adaptation, suggesting that the body’s natural thermoregulatory rhythm may help reduce thermal strain when adaptation occurs in the morning. This might open new possibilities and warrants further investigation into the effect of circadian rhythms on adaptation in indoor environmental settings