Institute of Higher Nervous Activity and Neurophysiology

Introduction. As a kind of sport, winter swimming differs from pool- or open-water swimming due to the extreme cold stress it places on the body. Swims in cold water with facial immersion trigger a diving reflex, which may be accompanied by the development of cardiac arrhythmias under low-temperature conditions. We believe that specific criteria should be developed for permitting individuals to engage in cold-water swimming. Objective. Study of the characteristics of changes in the state of the cardiovascular and nervous systems, autonomic regulation, and metabolic parameters in winter swimmers at water temperatures from +0.5 °C to +2.0 °C both over various standard distances and during a 10-min swim of 450–550 m. Materials and methods. Under competitive conditions and during training camps (at water temperatures from +0.5 °C to +1.5 °C and air temperatures from –15 °C to –18 °C), 24 winter swimmers were examined. These were 9 women aged 23–55 (mean age 39.1 ± 2.7 years); 3 men aged 71 ± 3 years; and 12 men aged 35–60 (mean age 43 ± 1.8 years), all of whom had received medical clearance for participation. The participants’ condition was analyzed before and after swims at distances of 25 m backstroke, 25 m and 200 m front crawl (freestyle), as well as after a 10-min swim of 400–450 m. The functional state of the body was assessed using 12-lead electrocardiography, blood pressure measurement, determination of capillary blood glucose concentration, and assessment of simple sensorimotor reaction to a light stimulus. Statistical analysis to identify differences between parameters was performed using the GraphPad Prism 8 software package for Windows 10. Results. A statistically significant increase in the mean capillary blood glucose levels was observed after swims of 25 m ( p < 0.05), 200 m ( p < 0.01), and 400 m compared to the baseline. An increase in the simple sensorimotor reaction time after 10-min swims was noted; thus, the baseline of 263 ± 10 ms vs. 328 ± 21 ms after the swim ( p < 0.01). According to ECG data, at baseline, 67% of the examined individuals showed a widened P wave > 0.11 ms. After short-distance swims of 25 m, P wave duration exceeded the upper limit of normal in 85% of the examined. Compared to backstroke swims, swims with facial immersion in water were accompanied by a significantly more pronounced widening of the P wave ( p < 0.05), slowing of atrial conduction PQ ( p < 0.01), and a more pronounced increase in the QTc interval ( p < 0.05). The QTc value progressively increased in accordance with the duration of the swims. During a 10-min swim, 50% of the examined individuals showed a QTc > 500 ms. Conclusions. Low water temperature is a factor that, even in cold-adapted athletes, provokes stress accompanied by an increase in blood glucose. During swims in cold water with facial immersion, the activated diving reflex triggers cardiac arrhythmias. During long-distance swims, conduction in the nervous system slows down, manifested in an increased simple sensorimotor reaction time and slowed myocardial conduction, reaching pathological values. In this regard, we believe that an additional criterion for medical clearance — assessment of the response of the cardiovascular system to cold-water immersion — should be developed and implemented for beginners wishing to engage in cold-water swimming, in order to avoid the risk of a pathological cardiovascular reaction.

The preliminary study aimed to identify connections between regions of the cerebral hemispheres based on the amplitude-amplitude coupling of theta and alpha rhythms during awakening with varying degrees of activity restoration. Out of 83 subjects, a group of 15 people was selected for analysis in studies of awakening from daytime sleep. The work used a bimanual psychomotor test, which allowed observation of the awakening process. Electroencephalography (EEG) was recorded from 18 silver chloride electrodes throughout the experiment. The amplitude characteristics of the EEG were evaluated using a continuous wavelet transform based on the “mother” Morlet wavelet. The Kendall correlation coefficient was used to measure the amplitude-amplitude coupling of the EEG rhythms. Upon awakening, the largest number of asymmetric connections in the interaction of theta and alpha rhythms were noted in the left frontotemporal area (connections with ten areas) and the left middle temporal area (connections with six areas), both within and between hemispheres, followed by complete restoration of activity (ten button presses with both the right and left hands). In cases of incomplete restoration of activity (from seven to nine button presses), asymmetric connections were observed upon awakening between the right middle temporal region and eight regions, as well as between the left region and four regions in terms of theta-alpha rhythm interaction. The left posterior temporal region connects to nine regions, while the right region connects to five regions. The awakening process with varying levels of recovery was characterized by differences in the number of intra- and interhemispheric connections between brain regions. The greatest number of intra- and interhemispheric alpha and theta rhythm connections were observed during incomplete recovery compared to complete recovery upon awakening.

The circadian phase difference between morning and evening types is a fundamental aspect of chronotype. However, results of categorizations into chronotypes based on reported sleep times show low concordance with those based on measurements of the hormonal or physiological or molecular rhythm-markers of the circadian phase. This might be partially explained by the profound individual differences in the phase angle between the sleep-wake cycle and these rhythms that depends on chronotype, age, sex, and other factors. Here, we examined the possibility of using self-reported sleep times in the condition of 5-days-on/2-days-off school/work schedule to estimate circadian phase differences between various chronotypes. In an in silico study, we determined that, for such an estimation, similarities of the compared chronotypes in weekend sleep duration and weekend-weekday gap and in risetime are required. In the following empirical and simulation studies of sleep times reported by 4940 survey participants, we provided examples of the estimation of circadian differences between chronotypes, and the model-based simulations of sleep times in morning and evening types exemplified a way to confirm such estimations. The results of in silico, empirical, and simulation studies underscore the possibility of using bedtimes and risetimes for direct estimation of the circadian phase differences between individuals in real-life situations, such as a 5-days-on/2-days-off school/work schedule. Additionally, the results of these studies on different chronotypes provided further mathematical modeling and empirical evidence for our failure to sleep more on weekends to recover/compensate/pay back/ catch up on lost sleep.

Morning and night rotating shifts cause misalignment of the sleep-wake cycle in relation to the biological night. We examined whether, to explain sleep timing and duration during and after these rotating shifts, it is necessary to propose the shifts of the circadian phase of sleep and disruption of its circadian control. A model of sleep-wake regulation was applied to simulate sleep times reported by German shiftworkers for 6 phases of the cycle of rotating shiftwork (days on and off for morning, evening, and night shifts). To simulate these sleep times, the assumptions of circadian phase shifts and loss of the permanent control of the circadian clocks over the sleep-wake cycle were not required. The simulations additionally predicted sleep times recently reported by Uruguayan and Norwegian shiftworkers for these 6 phases of shiftwork. Sleep deprivation during night shifts and irrecoverable loss of sleep prior to morning shifts can be responsible for the adverse health effects of rotating shiftwork. The extensions of the wake phase of the sleep-wake cycle before morning shifts and during night shifts can directly compromise the chronophysiological organization of the organism via disturbing the peripheral rather than central circadian clocks governing the sleep-wake cycle.