Background While A few studies have suggested associations between ambient temperature and cardiac autonomic function, the relationship between hourly temperature variations and heart rate variability (HRV) remains unclear.

Objective To examine the acute effects and lag patterns of short-term ambient temperature exposure on HRV at an hourly temporal resolution during cold and warm seasons, and to further characterize the exposure-response relationships.

Methods We conducted a longitudinal panel study involving 1047 eligible participants who ordered repeated 24 h ambulatory electrocardiogram monitoring at the First Affiliated Hospital of Xinjiang Medical University in Urumqi, Xinjiang Uygur Autonomous Region, China, between March 2020 and March 2022. Twelve HRV parameters were extracted, including standard deviation of all NN intervals (SDNN), standard deviation of the 5 min averages of NN intervals (SDANN), standard deviation of NN intervals index (SDNNIDX), root mean square of successive differences of adjacent NN intervals (rMSSD), triangle index (TI), percent of NN50 in the total number of NN intervals (pNN50), total power (TP), ultralow frequency (ULF), very low frequency (VLF), low frequency (LF), high frequency (HF), and ratio of low frequency to high frequency (LF/HF). Hourly meteorological data (mean temperature and relative humidity) were obtained from the nearest monitoring stations to participants' residences. Linear mixed effects models (LME) and generalized additive mixed effects models (GAMM) were employed to assess temperature-HRV associations and lag patterns during cold (from October to March next year) and warm (from April to September) seasons, with stratification by sex and age.

Results A total of 1047 Urumqi residents who had repeated 24 h ambulatory electrocardiograms were included in this study. In both cold and warm seasons, the exposure-response relationship between ambient temperature and HRV showed an approximately linear negative correlation with no threshold effect; HRV parameters decreased as ambient temperature increased. The correlation between ambient temperature and HRV usually began at a lag of 6 h and disappeared around 24 h lag. The cold season induced a stronger effect on HRV decline, and for every 1 °C increase in ambient temperature during the cold season from lag 0 to 6 h, SDNN decreased by 0.72%, SDNNIDX decreased by 1.46%, TI decreased by 0.07%, SDANN decreased by 1.23%, rMSSD decreased by 1.00%, pNN50 decreased by 2.74%, TP decreased by 3.30%, ULF decreased by 3.74%, VLF decreased by 3.76%, LF decreased by 2.75%, HF decreased by 2.49%, and LF/HF decreased by 0.74%; for every 1 °C increase in ambient temperature during the warm season, SDNN decreased by 0.56%, SDNNIDX decreased by 1.08%, TI decreased by 0.29%, SDANN decreased by 0.56%, rMSSD decreased by 0.68%, pNN50 decreased by 2.11%, TP decreased by 2.64%, ULF decreased by 3.14%, VLF decreased by 2.74%, LF decreased by 1.85%, HF decreased by 1.68%, and LF/HF decreased by 0.47%.

Conclusion Elevated ambient temperatures in Urumqi are significantly associated with decreased HRV in the residents, affecting cardiac autonomic function.