The temperature of the bedroom at night could affect heart health - particularly in older adults.
Heat places extra demands on the cardiovascular system, according to lead study author Dr. Fergus O'Connor from Griffith University in Queensland, Australia.
When the human body is exposed to heat, its reaction is to work harder to try and circulate blood to the skin surface for cooling, he noted.
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"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," O'Connor stated in a press release.
Researchers aimed to understand how real-world bedroom temperatures affected older adults.
The team followed 47 adults living in southeast Queensland averaging 72 years of age.
While many sleep observations are conducted in special clinics, this was a "free-living" study, meaning the participants carried on with their normal activities and sleep schedules.
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Scientists monitored the participants throughout an entire Australian summer, from December to March. Each person wore a high-tech fitness tracker to monitor their heart rate from 9 p.m. to 7 a.m., according to the release.
Sensors were then placed directly in participants' bedrooms to record the temperatures, monitoring over 14,000 nighttime hours of sleep in total.
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The temperature at which the heart began to show signs of disruption was a little more than 75 degrees Fahrenheit.
Between 75 and 79 degrees Fahrenheit, the odds of a "clinically relevant" drop in heart recovery rose by 40%, the researchers found.
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Between 79 and 82 degrees Fahrenheit, the odds doubled. Above 82, the risk was nearly triple compared to cooler rooms.
"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," O'Connor said.
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While the study shows a strong link between heat and heart stress, its observational design means that it doesn't definitively prove heat is the only cause, the researchers acknowledged.
As the study only focused on older adults in Australia, it may not apply to other populations.
Also, while the wearable devices are advanced, they are not as precise as the medical-grade ECGs used in clinical settings.
O'Connor emphasized a gap in temperature guidance - while there are guidelines for maximum daytime indoor temperature, there are no equivalent recommendations for nighttime conditions.
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The study was published in the journal BMC Medicine.
