Numerous researchers have suggested that climate change affects both the onset and severity of cardiovascular and mental disorders at a population level, but its impact on higher mental functions (HMF) and autonomic nervous system (ANS) remains relatively unexplored. In order to evaluate the HMF and the autonomic regulation of the cardiac activity we examined forty-five individuals (18 males u 27 females; 18.7 +/- 0.6 year old) in September 2017 and February 2018. Assessment of ANS was performed by heart rate variability (HRV) measurement, where we-minute HRV recordings by three-lead electrocardiography (Cardio, Ukraine) were obtained at rest in supine position and during cardiovascular reflex tests (active tilt test and deep breathing test) followed by analyzes of standard time and frequency domain HRV parameters. HMF here assessed by 4 standard tests: Schulte tables and Anisimov-Bourdon test to assess the characteristics of voluntary attention; "Wellbeing-Activity-Hood" questionnaire and State-Trait Anxiety Inventory for evaluation of psycho-emotional status. Correlation analyses among autumn and winter registrations revealed significant associations between parameters of voluntary attention with wind velocity in September and with most of the meteorological factors in February. Most of the associations with meteorological factors (temperature and pressure) and anxiety were revealed during observation that coincided with the heat wave event in September 2017. Identified associations between RRNN, RMSSD, pNN50, LF, VLF and humidity were suggestive for increased sympathetic activity during both cardiovascular reflex tests and these effects were similar in September and February. Negative correlation between the temperature and RRNN also indicates an increase in sympathetic activity as temperature increases during the deep breathing test. These results suggest a moderate prevalence of the sympathetic regulation of the cardiac activity with low emotional stability during heat wave exposure that could be considered as a potential risk factor of cardiovascular morbidity brought about by climate changes.