Effect of nighttime bedroom temperature on heart rate variability in older adults: an observational study.

BACKGROUND: Climate change is increasing the frequency of hot nights, which may contribute to cardiovascular morbidity and mortality by impairing sleep and autonomic recovery. Despite World Health Organization guidelines for maximum daytime indoor temperatures (26 °C, 79 °F), there are no equivalent recommendations for nighttime conditions. We investigated the impact of nocturnal bedroom temperature on heart rate and heart rate variability (HRV) in free-living older adults. METHODS: In this observational study, 47 community-dwelling adults aged ≥ 65 years in southeast Queensland, Australia, were monitored across one summer (December 2024-March 2025). Wearable devices recorded heart rate and HRV during nighttime periods of sleep between the hours of 9 PM-7 AM, while in-home sensors continuously measured bedroom temperature. The primary outcome was the natural logarithm of the root mean square of successive differences (lnRMSSD). Secondary outcomes were log-transformed frequency-domain HRV indices (high frequency: lnHF, low frequency: lnLF, low to high frequency ratio: lnLF:HF) and heart rate. Generalised mixed effects models analysed associations between wearable derived outcomes and temperature categories (< 24 °C [79 °F], 24-26 °C [75-79 °F], 26-28 °C [79-82 °F], 28-32 °C [82-90 °F]). Clinically relevant thresholds were defined as ≥ 1.5 standard deviation change in HRV or ≥ 5 beats·min⁻1 change in heart rate. RESULTS: Across 14,179 valid nighttime hours, median bedroom temperature was 25.9 °C (Q1, Q3; 24.6, 26.9). Compared with < 24 °C (79 °F), nighttime bedroom temperatures of 24-26 °C (75-79 °F; odds ratio: 1.4; 95% confidence interval [1.2, 1.6], P < 0.001), 26-28 °C (79-82 °F; 2.0 [1.8-2.3]) and 28-32 °C (82-90 °F; 2.9 [2.5-3.4]) were associated with greater odds of clinically relevant reductions in lnRMSSD (P < 0.001). Higher temperatures were also linked to reduced lnHF and lnLF, increased ln(LF:HF), and elevated heart rate (all P < 0.001). CONCLUSIONS: Nocturnal bedroom temperatures above 24 °C (79 °F) were associated with greater likelihood of autonomic disruption and increased heart rate in older adults, consistent with a shift toward sympathetic dominance and heightened physiological stress, with greater effects observed as temperature increased. These findings provide real-world physiological evidence supporting the development of nighttime indoor temperature guidelines to protect vulnerable populations in a warming climate.