Climate change has increased ambient temperatures, providing a serious health risk, specifically for older adults with compromised thermoregulation. Most research highlights the dangers of extreme daytime heat, while the impact of elevated nighttime temperatures has received less attention. Evidence suggests that hot nights can increase morbidity and mortality by disrupting sleep and stressing the cardiovascular system. Climate models predict that by the end of the century, more heat-related deaths may result from hot nights than hot days. Current public health recommendations from the World Health Organization largely ignore nighttime temperature guidelines. This oversight is concerning, given the potential health impairments associated with hot nights, like oxidative stress and autonomic dysregulation, which can worsen with persistent exposure and inadequate recovery.
The aim of this study was to assess the link between nighttime bedroom temperature and autonomic cardiovascular responses in free-living older adults over an entire summer season. This study quantified the changes in heart rate and heart rate variability (HRV), established surrogate markers of autonomic nervous system function in relation to increased nighttime indoor temperatures, and sought to detect a temperature threshold associated with clinically meaningful physiological changes. This study aimed to address a critical gap between epidemiological evidence, mechanistic understanding, and current heat health guidance.
Adults aged 65 years and older living in southeast Queensland, Australia, were recruited for an observational study conducted between December 1, 2024, and March 17, 2025, after approval from Griffith University Human Research Ethics Committee. Continuous heart rate and HRV data were collected using a photoplethysmography-based wearable device worn on the non-dominant wrist. HRV indices were derived during detected sleep periods and included the root mean square of successive differences (RMSSD) and frequency-domain measures (low frequency [LF], high frequency [HF], and LF: HF ratio). Indoor environmental conditions were concurrently monitored with calibrated sensors placed in many rooms of each participant’s home, including the main bedroom, recording temperature and humidity at 10-minute intervals. HRV data were restricted to nighttime hours (9 PM to 7 AM), artefact-free recordings (<5%), and were natural log-transformed to ensure normality
Statistical analyses used linear and generalized mixed effects models to account for repeated measures within individuals via random intercepts. The primary outcome was nighttime lnRMSSD, and secondary outcomes include lnHF, lnLF, ln(LF: HF), and heart rate. Nighttime bedroom temperatures were categorised into five exposure ranges (below 24 °C, 24 to 26 °C, 26 to 28 °C, 28 to 32 °C, and above 32 °C). Clinically relevant outcomes were defined as a ≥1.5 standard deviation reduction in lnRMSSD and a ≥5 beats·min⁻¹ change in heart rate relative to individual normative values. Sensitivity analyses excluded participants taking medications known to affect thermoregulation or heat sensitivity.
Of the 79 individuals screened, 47 participants (32 women, median age 72 years [68-77]) completed the study. Median nighttime bedroom temperature was 25.9 °C. Increasing nighttime bedroom temperature was significantly associated with reduced lnRMSSD, lnHF, and lnLF, increased ln(LF: HF) ratios, and elevated heart rate (all P < 0.001). This indicates progressive autonomic imbalance and increased physiological strain. The odds of experiencing clinically meaningful reductions in lnRMSSD increased in a dose-response manner: 24 to 26 °C (odds ratio [OR]: 1.4, 95% confidence interval [CI]: 1.2 to 1.6), 26 to 28 °C (OR: 2.0, 95% CI: 1.8 to 2.3) and 28 to 32 °C (OR: 2.9, 95% CI: 2.5 to 3.4) all P < 0.001. Similar temperature-related effects were observed for heart rate and ln(LF: HF), and reduced for lnHF and lnLF, with effects evident above 24 °C and more pronounced beyond 26 °C. These links were not materially altered by medication use.
This study provides novel real-world evidence that elevated nighttime bedroom temperatures are linked with autonomic disruption and increased cardiovascular strain in older adults. The results suggest that nighttime temperatures above 24°C, particularly above 26 °C, impair nocturnal autonomic recovery and may increase heat-related health risk. These findings underscore the need for explicit nighttime indoor temperature guidelines and targeted interventions to protect vulnerable populations as hot nights become more frequent under climate change.
Reference: O’Connor FK, Bach AJE, Forbes C, et al. Effect of nighttime bedroom temperature on heart rate variability in older adults: an observational study. BMC Med. 2025;23:703. doi:10.1186/s12916-025-04513-0
