Skin Temperature Dynamics during Sleep Onset Latency under Different Ambient Temperatures
The bedroom thermal environment is a key determinant of sleep onset, yet most studies manipulate ambient temperature and evaluate sleep onset latency (SOL) between conditions. Continuous skin-temperature dynamics during the SOL, particularly across bedroom temperatures, remain poorly characterised. This study examined skin-temperature dynamics before objectively defined sleep onset across multiple bedroom temperatures. Twelve healthy adults (6 males, 6 females; 23–42 years; BMI 17.7–32.9 kg/m²) completed four overnight sessions at 22°C, 24°C, 26°C, and 28°C. From 23:00 lights-out to 07:00 lights-on, skin temperatures at nine sites were recorded every 30 seconds, while polysomnography was continuously monitored. Sleep stages were scored according to the American Academy of Sleep Medicine manual, and the first non-wake epoch defined sleep onset. Temperature series were aligned to this point, and distal (DST), proximal (PST), and mean skin temperatures (MST) were derived. The distal–proximal gradient (DPG) was calculated as the difference between distal and proximal temperatures. Linear mixed-effects models with ambient temperature and SOL segment (early: −30 to −15 min; late: −15 to 0 min) as fixed factors showed that ambient temperature strongly affected DST, MST, and DPG in the early period of SOL, but effects were markedly attenuated in the late period. Across all four bedroom temperatures, DST, MST, and DPG converged toward similar levels as sleep onset approached. These findings suggest that the body is not passively constrained by ambient temperature but actively adjusts skin temperature and heat dissipation through distal thermoregulation to reach a relatively stable sleep-conducive state.