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Despite World Health Organization guidelines for maximum daytime indoor temperatures (26\u00a0\u00b0C, 79 \u00b0F), 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.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods<\/jats:title>\n                    <jats:p>\n                      In this observational study, 47 community-dwelling adults aged\u2009\u2265\u200965\u00a0years in southeast Queensland, Australia, were monitored across one summer (December 2024\u2013March 2025). Wearable devices recorded heart rate and HRV during nighttime periods of\u00a0sleep between the hours of\u00a09 PM\u20137 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\u00a0frequency-domain HRV indices (high frequency: lnHF, low frequency: lnLF, low\u00a0to high frequency ratio: lnLF:HF) and heart rate. Generalised mixed effects models analysed associations between wearable derived outcomes and temperature categories (&lt;\u200924\u00a0\u00b0C [79 \u00b0F], 24\u201326\u00a0\u00b0C [75\u201379 \u00b0F], 26\u201328\u00a0\u00b0C [79\u201382 \u00b0F], 28\u201332\u00a0\u00b0C [82\u201390 \u00b0F]). Clinically relevant thresholds were defined as\u2009\u2265\u20091.5 standard deviation change in HRV or\u2009\u2265\u20095 beats\u00b7min\u207b\n                      <jats:sup>1<\/jats:sup>\n                      change in heart rate.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>\n                      Across 14,179 valid nighttime hours, median bedroom temperature was 25.9\u00a0\u00b0C (Q1,\u00a0Q3; 24.6,\u00a026.9). Compared with\u2009&lt;\u200924\u00a0\u00b0C (79 \u00b0F), nighttime bedroom temperatures of 24\u201326\u00a0\u00b0C (75\u201379 \u00b0F; odds ratio: 1.4; 95% confidence interval [1.2, 1.6],\n                      <jats:italic>P<\/jats:italic>\n                      \u2009&lt;\u20090.001), 26\u201328\u00a0\u00b0C (79\u201382 \u00b0F; 2.0 [1.8\u20132.3]) and 28\u201332\u00a0\u00b0C (82\u201390 \u00b0F; 2.9 [2.5\u20133.4]) were associated with greater odds of clinically relevant reductions in lnRMSSD (\n                      <jats:italic>P<\/jats:italic>\n                      \u2009&lt;\u20090.001). Higher temperatures were also linked to reduced lnHF and lnLF, increased ln(LF:HF), and elevated heart rate (all\n                      <jats:italic>P<\/jats:italic>\n                      \u2009&lt;\u20090.001).\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusions<\/jats:title>\n                    <jats:p>Nocturnal bedroom temperatures above 24\u00a0\u00b0C (79 \u00b0F) 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.<\/jats:p>\n                  <\/jats:sec>","DOI":"10.1186\/s12916-025-04513-0","type":"journal-article","created":{"date-parts":[[2025,12,29]],"date-time":"2025-12-29T16:26:20Z","timestamp":1767025580000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":17,"title":["Effect of nighttime bedroom temperature on heart rate variability in older adults: an observational study"],"prefix":"10.1186","volume":"23","author":[{"given":"Fergus K.","family":"O\u2019Connor","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Aaron J. 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