Higher bedroom temperatures at night are associated with lower heart rate variability and higher heart rate during sleep.
Climate projections suggest hot nights may soon contribute more to heat-related deaths than hot days, yet indoor nighttime thresholds remain undefined. To address this gap, researchers tracked older adults in their own homes across summer, pairing bedroom temperature with wearable measures of cardiovascular strain during sleep.
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Researchers monitored 47 community-dwelling adults aged 65 years and older in southeast Queensland, Australia, from December 2024 to March 2025. Participants wore a wrist device that used photoplethysmography, an optical method that estimates pulse timing from blood flow changes, to derive sleep-period heart rate and heart rate variability. In-home sensors continuously measured bedroom temperature, and analyses focused on nighttime sleep hours, 9 PM to 7 AM, comparing temperature bands and clinically meaningful changes in physiology.
- Compared with nights below 24°C (75.2°F), warmer bedrooms made substantial reductions in a measure of heart rate variability that reflects the small changes in the timing between one heartbeat and the next progressively more likely. At 28 to 32°C (82.4 to 89.6°F), participants were nearly three times more likely to show this reduction.
- The decline was not limited to these beat-to-beat changes. It also affected the repeating rises and falls in heart rate that unfold over several seconds and can be separated into a faster component linked to breathing and a slower component linked to blood pressure regulation during sleep.
- The researchers compared these repeating slower and faster fluctuations by taking their ratio. This ratio rose at higher temperatures, a pattern often interpreted as greater sympathetic ("fight-or-flight") influence, although that interpretation remains debated.
- Sleep heart rate increased as bedrooms warmed. At 28 to 32°C, participants were nearly four times more likely to experience an increase of at least 5 beats per minute during sleep compared with nights below 24°C.
These patterns connect plausibly to thermoregulation, the body's process for keeping internal temperature stable. Heat exposure can increase skin blood flow for cooling, which raises cardiac workload and can elevate heart rate. Reduced heart rate variability reflects diminished parasympathetic ("rest-and-digest") activity. Together, the pattern suggests that warmer bedrooms may impair overnight recovery of the autonomic nervous system and increase physiological strain.
The study included a relatively small sample from a single subtropical region, and it was observational, meaning it cannot prove causation. Still, the findings suggest that keeping bedrooms cooler than 24°C at night could represent a practical strategy to help reduce cardiovascular strain in older adults. In episode #107, Dr. Michael Grandner, a renowned sleep expert, explains how to overcome insomnia and use sleep to optimize health.