An air conditioner unit. [Yonhap]
An air conditioner unit. [Yonhap]

Turning off the air conditioner at night to avoid high electricity bills during tropical nights may not benefit your health, new research suggests. Older adults in particular face a greater risk of elevated heart rates and reduced cardiac autonomic recovery when sleeping in warm bedrooms, the study found.

According to a paper published Friday in the international journal BMC Medicine, researchers at Griffith University in Australia studied 47 adults aged 65 and older living in southeastern Queensland between Dec. 1, 2024, and March 17, 2025. The median age of participants was 72.

Participants wore wrist-based wearable devices throughout the study period. Researchers measured heart rate and heart rate variability, or HRV, during hours identified as sleep between 9 p.m. and 7 a.m. the following morning, while bedroom temperature was recorded at 10-minute intervals using sensors installed in the room. The total nighttime data used in the analysis amounted to 14,179 hours, and the median bedroom temperature over the study period was 25.9 degrees Celsius.

The research team divided bedroom temperatures into four ranges — below 24 C, 24–26 C, 26–28 C and 28–32 C — and compared cardiac autonomic responses across each band.

The analysis found that as bedroom temperature rose, the likelihood of a clinically meaningful drop in lnRMSSD, an HRV indicator, increased.

Using below 24 C as the baseline, the odds ratio was 1.4 times higher in the 24–26 C range and 2.0 times higher in the 26–28 C range, climbing to 2.9 times in the 28–32 C range. Nighttime heart rate also tended to rise as bedroom temperature increased.

The researchers defined a "clinically meaningful change" as a drop in HRV of more than 1.5 standard deviations below an individual's usual level, or a shift in heart rate of five or more beats per minute.

However, the "2.9 times" figure does not mean cardiac function deteriorated 2.9-fold or that the risk of a heart attack increased by that amount. It means the odds of lnRMSSD falling significantly below the researchers' defined threshold were 2.9 times higher — and it should not be interpreted as an equivalent increase in the actual probability of such an event occurring.

Heart rate variability refers to the tiny fluctuations in time between heartbeats. The RMSSD measure used in this study is a well-established indicator of how actively the parasympathetic nervous system — which calms the heart — is functioning. Lower values suggest the body may not have relaxed sufficiently overnight and that its recovery capacity may have been diminished.

In hot conditions, the heart must pump more blood to the skin to dissipate body heat, increasing cardiac workload. The Korea Disease Control and Prevention Agency notes that heat activates the sympathetic nervous system and raises heart rate. When dehydration from sweating compounds that strain, blood can thicken, placing even greater pressure on the cardiovascular system. Older adults and those with cardiovascular disease may be especially vulnerable to these changes.

The findings should not be applied universally, the researchers cautioned. The study was limited to 47 adults aged 65 and older living in the hot, humid subtropical climate of Queensland, Australia. Whether the same results would hold for other age groups or climates has not been confirmed.

The measurement approach also has limitations. The team estimated sleep periods and HRV using algorithms from wrist-worn wearable devices and did not track how much participants used fans or natural ventilation. The study also did not follow participants to determine whether actual risks of heart attack, stroke or death increased. The researchers said they focused on nighttime hours not to assess sleep quality per se, but to minimize the influence of daytime activity and examine the relationship between bedroom temperature and autonomic nervous system responses.

The researchers suggested that, where possible, keeping older adults' bedroom temperatures below 24 C at night could be beneficial. They noted, however, that this recommendation is based on their study alone and does not constitute an official sleep-temperature guideline set by domestic or international health authorities.

South Korea's Ministry of Interior and Safety recommends keeping the gap between indoor and outdoor temperatures at around 5 C when using air conditioning in summer, and advises an indoor cooling temperature of 26–28 C for health reasons.

The two standards serve different purposes. The ministry's 26–28 C guideline is a general household rule that balances protection against both heat and air-conditioning sickness, while the study's 24 C threshold was derived from observing bedroom temperature and cardiac autonomic responses specifically in older adults. The two figures are therefore difficult to compare directly, and it would be premature to conclude that one is more correct than the other.

Experts advise against fixating on a single number, saying the more important thing during a heat wave is to use cooling in a way that suits one's physical condition. For those without air conditioning, minimizing sunlight entering the home during the day and ensuring cross-ventilation at night can help. For those using air conditioners, it is advisable not to set the temperature too far below the outdoor temperature and to ventilate the room periodically.

If symptoms such as sudden chest pain, chest pressure, difficulty breathing, cold sweats or reduced consciousness occur during the night, they should not be dismissed as mere heat discomfort — call 119 immediately for emergency care. Older adults and those with cardiovascular conditions are particularly at risk of accumulating sleep deprivation and cardiovascular strain when tropical nights persist for several consecutive days, making it important to secure adequate recovery time through appropriate cooling and sufficient hydration rather than enduring the heat.


rainbow@heraldcorp.com