<p>Few studies have investigated the association of exposure to PM_2.5 at the individual level on sleep quality and next-day physical performance, which are both important for human health. To fill the gap, this field study was conducted among 183 young adults who were required to participate in standardized physical fitness test which objectively evaluated their physical performance, with their bedroom environment and sleep quality of the night prior to the fitness assessment were continuously monitored. Multiple linear regression was used to analyse the association between the bedroom environment, sleep quality and next-day physical performance, and to examine the interaction effects of environmental factors. The results show that PM_2.5 was significantly associated with a reduction in both the proportion of deep sleep and the next-day performance of a long-distance running test. Its negative association with long-distance running performance was exacerbated by a high CO₂ level (3,961 ppm) during sleep. These findings suggest the importance of good bedroom air quality (low levels of PM_2.5 and CO₂) for maintaining occupants’ sleep quality and health. For young people, maintaining a clean and well-ventilated sleeping environment that minimizes indoor particulate matter exposure is essential for ensuring good sleep quality and safeguarding physical health.</p>