New research suggests that older adults experience lower stress levels when their bedroom is kept at 24°C (75 °F) during sleep.
Maintaining a bedroom temperature around 24°C (75.2°F) or lower during sleep may help older adults avoid heat-related physiological strain, according to new Griffith University research tracking real-world summer bedroom conditions and wearable heart metrics.
Dr. Fergus O’Connor from Griffith’s School of Allied Health, Sport and Social Work assessed how warmer nighttime bedroom temperatures affect heart rate and stress-related autonomic responses in older adults living at home.
“For individuals aged 65 years and over, maintaining overnight bedroom temperatures at 24°C (75.2°F) reduced the likelihood of experiencing heightened stress responses during sleep,” Dr. O’Connor said.
How Heat Affects the Heart During Sleep
The study focused on how the body responds to heat during rest, particularly in older adults who may be more vulnerable to temperature-related stress and less able to recover overnight.
“When the human body is exposed to heat, its normal physiological response is to increase the heart rate,” Dr. O’Connor said.
“The heart is working harder to try and circulate blood to the skin surface for cooling. However, when the heart works harder and for longer, it creates stress and limits our capacity to recover from the previous day’s heat exposure.”
Researchers used heart rate variability (HRV)—a common marker of autonomic nervous system recovery—to capture physiological “stress response” signals during sleep. Lower HRV generally indicates reduced parasympathetic (“rest-and-digest”) activity and greater physiological strain.
Real-World Monitoring Over Summer
Unlike short laboratory experiments, this was an in-home observational study of 47 community-dwelling adults aged 65+ in southeast Queensland, Australia, monitored across one summer (December 2024–March 2025).
Participants wore Fitbit Inspire 3 trackers on their non-dominant wrist, while bedroom temperatures were recorded continuously using installed temperature sensors (with 10-minute environmental sampling). For analysis, the team focused on nighttime sleep hours between 9 PM and 7 AM.
Across 14,179 valid nighttime hours, the median bedroom temperature was 25.9°C (78.6°F) (interquartile range 24.6–26.9°C [76.3–80.4°F]), meaning many participants spent substantial time above the threshold linked to healthier autonomic recovery.
Hotter Bedrooms, Clearer Signs of Autonomic Disruption
The researchers grouped bedroom temperatures into ranges and compared outcomes against the coolest category (<24°C [<75.2°F]). As nighttime temperatures increased, the likelihood of clinically relevant autonomic disruption rose in a stepwise pattern.
Compared with <24°C (<75.2°F), bedroom temperatures were associated with greater odds of a clinically meaningful reduction in the primary HRV measure (lnRMSSD):
- 24-26°C (75.2-78.8°F): odds ratio 1.4
- 26-28°C (78.8-82.4°F): 2.0
- 28-32°C (82.4-89.6°F): 2.9
Higher nighttime temperatures were also linked to elevated heart rate, reduced HRV in both high- and low-frequency components, and shifts consistent with a move toward greater sympathetic (“fight-or-flight”) dominance—a physiological pattern associated with reduced overnight recovery.
“When the human body is exposed to heat, its normal physiological response is to increase the heart rate,” Dr. O’Connor said.
“The heart is working harder to try and circulate blood to the skin surface for cooling.However, when the heart works harder and for longer, it creates stress and limits our capacity to recover from the previous day’s heat exposure.”
Study participants wore fitness activity trackers on their non-dominant wrist, and the bedroom temperature was monitored via installed temperature sensors throughout the Australian summer-long data collection period.
Why It Matters as Nights Get Hotter
“Climate change is increasing the frequency of hot nights, which may independently contribute to cardiovascular morbidity and mortality by impairing sleep and autonomic recovery,” Dr. O’Connor said.
“While there are guidelines for maximum daytime indoor temperature, 26°C (78.8°F), there are no equivalent recommendations for nighttime conditions.”
Reference: “Effect of nighttime bedroom temperature on heart rate variability in older adults: an observational study” by Fergus K. O’Connor, Aaron J. E. Bach, Connor Forbes, Shannon Rutherford, Sebastian Binnewies, Surendran Sabapathy and Norman R. Morris, 29 December 2025, BMC Medicine.
DOI: 10.1186/s12916-025-04513-0
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2 Comments
“Climate change is increasing the frequency of hot nights, which may independently contribute to cardiovascular morbidity and mortality by impairing sleep and autonomic recovery,”
It is generally agreed that the Earth has been warming about 0.16 degrees C per decade for the last several decades, albeit it is probably warming a little more at night. I’ll allow that it is also probably warming more at the poles, which are hard pressed to get to the sleeping-termperature category that is coolest. To account for those considerations, I’ll round that warming up to 0.20 degrees per decade. Over the 8 decades I have lived, that means nighttime Summer temperatures have increased about 1.6 degrees C, certainly less than the 2 degree range for the bottom two categories presented in the table. and far less than the seasonal changes experienced a century ago.
This seems to be an example of the ‘Chicken Little Syndrome’ where the researcher(s) worry excessively about trivial changes that are difficult to even measure accurately. How does a couple of hours of extra warmth translate to increased Heart Rate Variability, and more importantly, morbidity? Does it matter where in the sleep cycle the extra warmth occurs? Is the effect linear or cumulative? How many months are taken off an expected lifespan if the subjects adopted air conditioning in their 70s rather than in their 60s? So many questions and so few applicable answers!
75°F? That sounds super hot!
I have my bed room set to 65°F to 69°F, even during the winter.
One night I even slept with the AC set to 61°F and I did not even notice. And I sleep alone, in the nude, and with nothing more than a light blanket.