PHOTOPERIODIC CHRONOTYPE STABILITY IN STUDENTS OF KHANTY-MANSI AUTONOMOUS AREA – YUGRA AND VLADIKAVKAZ

. The aim of the work is to establish the peculiarities of the organization of various aspects of life in the dynamics of the winter/summer seasons among students studying in the conditions of the photoperiod of the northern region (Khanty-Mansi Autonomous Area – Yugra) and the temperate climatic zone (Vladikavkaz). In the Khanty-Mansi Autonomous Area, among students of both sexes, the proportion of persons with evening chronotypes was higher than in Vladikavkaz. Among thenorthernyoung men, there were no persons with a definite morning chronotype, and among the Vladikavkaz students there was noone with a definite evening chronotype; in the southern region, the share of “larks” was higher. In the dynamics of winter/summer in all groups, an increase in the proportion of “larks” was found, less pronounced in young men. Among the students of Vladikavkaz in both seasons, young men predominated among morning chronotypes, the percentage of persons with an arrhythmic chronotype was higher among young women. The values of the index of photoperiodic stability , in general, characterize the chronotype of the examined students as rigid. In the organization of life in the social group of students, social generators of rhythm prevail over natural factors. The study involved 153 students studying in the Khanty-Mansi Autonomous Area – Yugra (young women n=100, young men n=53), and 121 students from Vladikavkaz (young women n=83, young men n=38). The dynamics of the chronotype was investigated using the mobile application “Index of photoperiodic stability” [14] in 2020-2021. The mobile application includes the Horneand Ostbergquestionnaire, to which the respondents answer the questions in two versions: taking into account the preference of the time intervals for various activities in the winter and summer seasons of the year. The results of the Horneand Ostbergquestionnaire are interpreted as follows: the scores 16-30 – definitely evening type (DET), 31-41 – moderate evening type (MET), 42-58 – arrhythmic (intermediate) chronotype (AT), 59-69 – moderate morning chronotype (MMT), 70-86 – definitely morning chronotype (DMT). data processing. The mobile application “Index of photoperiodic stability” a computer program “Calculator of photoperiodic stability of the chronotype” individual calculations for subjective assessment of the chronotype with the most contrasting parameters of the photoperiod (winter and summer).

Introduction. Illumination is one of the most natural and definite geophysical characteristics of the territory [1]. The circadian rhythm of illumination varies greatly with the season of the year, except for the tropics. At high latitudes, the ratio of light and dark time of day undergoes significant changes throughout the year: in winter, dark prevails, in summer -light, which does not meet the physiological needs of humans [1]. According to I.I. Zamorsky et al., the need to adapt to the pronounced daily and annual dynamics of illumination led to the emergence of a functional system that synchronizes chronorhythms. Circadian rhythms are organized by the circadian system and circannual rhythms by the circannual system. Since these systems include the same structures, it was proposed to combine them into a chronoperiodic system [2]. The rhythm of physiological functions is one of the qualities of living matter [3][4][5]. The state of the chronobiological structure of the body depends on the light regime. Thus, under the conditions of a changed seasonal photoperiod, an inconsistency in the spatiotemporal perception of the chronotype was revealed in patients with psoriasis living in the Khanty-Mansi Autonomous Area -Yugra [6]. Chronobiological features of the structure of exacerbations of depressive states in the northern region have been established, the influence of desynchronosis of biological rhythms on the pathogenetic mechanism of development and dynamics of exacerbations has been revealed [7]. The ecophysiological role of the photoperiod in female reproductive function was investigated by changing the synthesis of melatonin, which controls its activity, according to the dynamics of conception [8].
The preservation of the chronostructure of functions is a universal criterion for the functional state of an organism. The chronotype refers to the individual characteristics of the organization of circadian rhythms. In 1976, based on the modification of the questionnaire by O. Öquist, J. Horne and O. Ostberg developed a test that is widely used at present to determine the chronotype (Horneand Ostberg Morningness-Eveningness Questionnaire, MEQ) [9]. The biorhythmological stereotype affects the body's ability to adapt [10], depends on endogenous and exogenous parameters [11].
Proceeding from the fact that the illumination mode acts as a natural generator of biorhythms, the aim of the study was to establish the peculiarities of the organization of various aspects of life in the dynamics of the winter/summer seasons among students studying in the conditions of the photoperiod of the northern region (Khanty-Mansi Autonomous Area-Yugra) and the temperate climatic zone (Vladikavkaz).
Objects and research methods. Khanty-Mansiysk Autonomous Area -Yugra is located in the center of the West Siberian Plain, between 58° and 62° north latitude, according to natural and climatic factors it is equated to the regions of the Far North. The climate varies from temperate to sharply continental [12; 13]. On the territory of the Khanty-Mansi Autonomous Area, significant changes in illumination are traced: the shortest daylight hours are recorded in the second half of December (05 hours 32 minutes), the largestin the second half of June (more than 19 hours). Vladikavkaz city is located at 43°01′00″N, 44°41′00″E, the climate is moderately continental. The length of the day on the winter solstice (December, 22) is 08 hours 59 minutes, in the second half of June -15 hours 22 minutes. Thus, the seasonal range of daylight hours in the Khanty-Mansiysk Autonomous Area -Yugra is much greater than in Vladikavkaz.
The study involved 153 students studying in the Khanty-Mansi Autonomous Area -Yugra (young women n=100, young men n=53), and 121 students from Vladikavkaz (young women n=83, young men n=38). The dynamics of the chronotype was investigated using the mobile application "Index of photoperiodic stability" [14] in 2020-2021. The mobile application includes the Horneand Ostbergquestionnaire, to which the respondents answer the questions in two versions: taking into account the preference Statistical data processing. The mobile application "Index of photoperiodic stability" [14] contains a computer program "Calculator of photoperiodic stability of the chronotype" [15], which makes individual calculations for subjective assessment of the chronotype with the most contrasting parameters of the photoperiod (winter and summer).
The program "Calculator of photoperiodic stability of the chronotype" calculates the Photoperiodic Stability of the Chronotype (PSC) by the formula: Σwi -total points when answering 19 questions of the Horne and Ostberg questionnaire for morning-evening preferences in winter; Σsi -total points when answering 19 questions of the Horne and Ostbergquestionnaire for morning-evening preferences in summer; di -difference in points in summer and winter for i questions; Σ|di| -sum of the absolute differences in points in summer and winter for all questions.
Normal values of photoperiodic stability of the chronotype are in the range of 4 conditional unit (cu) (5% probability) up to 16 cu (95% probability). When PSC<4 cu the chronotype is rigid, without obvious adaptive behavioral responses to exogenous factors. PSC>16 cu characterizes the chronotype as labile, significantly dependent on external influences. The study is single-step cross-sectional study. The sampling method is non-randomized. Descriptive statistics are represented by mean value (M) and mean error (m). Excel 2013 was used for statistical data processing. Data is presented in percent (%).
Research results and discussion. The data on the representation of various chronotypes in populations differ significantly. So, in the studies of N.A. Aghajanyan et al. (2013) established the following approximate frequency of distribution of chronotypes: 15%morning type, 20%eveningtype and 65%arrhythmic type [3].
According to other sources, the number of "larks" ranges from 15 to 25% [16]. In a study with the participation of lyceum students from Votkinskcity (the Udmurt Republic), more than half were "doves", the share of "larks" was quite significantabout a third of those surveyed; the smallest was the representation of "owls" [17].
On the territory of the Khanty-Mansiysk Autonomous Area -Yugra, a different representation of chronotypes is revealed: the percentage of "larks" is significantly reducedto 2-4%, with an increase in the contribution of persons with the evening chronotype to 38-40%; representatives of the arrhythmic (intermediate) chronotype prevail (56-58% and more) [10; 18; 19].
In our study, the analysis of the data obtained showed that there are differences in the representation of different chronotypes depending on the geographic location of the place of residence (tabl. 1). As it turned out, among the students of both genders in the Khanty-Mansi Autonomous Area -Yugra, which is characterized by a significant off-season range of the ratio of daylight to dark, there was a greater proportion of persons with evening chronotypes (%). So, in total (DET + MET) in the winter/summer season among the northern young men, there were 33.9%/17.0% of "owls" against, respectively, 21.0%/13.1% among students of Vladikavkaz, in the absence of persons with DET among the latter. In the women's groups, the representation of evening chronotypes in winter/summer was, respectively, 35.0%/32.0% among the northern young women and 19.2%/16.8% among female students of Vladikavkaz. The total ratio of the representation of young people with the morning chronotype (MMT + DMT) was different. Among the young men of the Khanty-Mansi Autonomous Area -Yugra, the percentage of larks was 5.7%/9.4%, respectively, in winter/summer, in the absence of persons with a definitely morning chronotype. Among Vladikavkaz male students, the proportion of morning types was significantly higher: 21.0%/23.7%. In the groups of young women, the differences were less pronounced: 6.0%/14.0% among the northern young women and 6.0%/10.8% among female students from Vladikavkaz.
In the study of I.N. Konareva (2016), with the participation of school-age children from Simferopol, the dynamics of the representation of persons with different chronotypes in the schooling process was discovered: the percentage of "larks" and "doves" decreased with age, "owls" were found starting from the age of 13. The author suggests that the reason is a rigid daily routine that does not allow younger students to focus on their internal biological needs. The biorhythmological characteristics of "doves" are more consistent with the organization of the educational process [20]. At the same time, the level of cortisol after waking up in the morning type is significantly higher than that of the evening type [21]. In a study with the participation of residents of Omsk city at the age of 18-35 years, representatives of the morning chronotype were mainly female, and the evening chronotype were male [22]. In our study, the share of students with an arrhythmic chronotype among northern young men in winter/summer was 60.4%/73.6% and 57.9%/63.1% among Vladikavkaz students, among young women, respectively, 59.0%/54.0% and 74.7%/72.3%.
Thus, the representation of chronotypes among students differed depending on the geographical characteristics of the place of residence. Representatives of the arrhythmic chronotype are leading in number, with a pronounced positive winter/summer dynamics in young men of the Khanty-Mansi Autonomous Area and a slight decrease in the proportion of young women in both groups. In the male groups of the northerners in both seasons of the year there were no persons with a definitely morning chronotype, among the Vladikavkaz youths there were no persons with a definitely evening chronotype. Differences related to gender were also found. Thus, among the students of Vladikavkaz in both seasons, young men prevailed among the "larks", and among the representatives of the arrhythmic chronotype, the percentage of young women was higher. In general, there was a tendency towards an increase in the manifestations of the morning chronotype in the summer season of the year for students of both territories, with its less pronounced in young men.
The photoperiodic stability of the chronotype (PSC) was calculated (tabl. 2). In the general groups (included all the presented chronotypes) in young men, this indicator was 2.89±0.72 cu northerners and 2.58±0.64 cu among the students of Vladikavkaz, that is, it differed slightly geographically and corresponded to a rigid type, independent of external factors. In the young women of the Khanty-Mansi Autonomous Area -Yugra, the average PSC value was within the normal range -4.19±1.02 cu, among the female students of Vladikavkaz corresponded to the rigid type -3.67±0.91 cu.
Analysis of PSC values by chronotypes did not reveal general patterns in terms of territorial and gender characteristics (tabl. 2). So, the lower limits of normal values, which correspond to natural physiological reactions to changes in the ratio of dark and light time of day, in the male groups of northerners corresponded to the average PSC values in larks, and in Vladikavkaz students the indicators of persons with an arrhythmic chronotype were closest to them. In both female groups, the doves showed normal reactivity.  Gubin and S.N. Kolomeichuk (2019) suggested that the lengthening of the dark period of the day is more unfavorable for the representatives of the evening chronotype, due to the even greater inhibition of the biological clock, and the opposite thesis is correct for the morning chronotypes, since the addition of the light phase accelerates the biological time [23].
The authors point to a deficiency or excess of a synchronizing factor, such as light, temperature and food intake, as one of the reasons for the decline in vitality. At the same time, there is no clarity about the optimal duration of the light phase and the frequency of food intake at the moment.
Conclusion. Thus, territorial and sex differences were found in the representation of chronotypes: in the Khanty-Mansi Autonomous Area -Yugra, characterized by a changed photoperiod, the proportion of persons with evening chronotypes was greater than in Vladikavkaz, among students of both sexes. For the male groups, the prevalence of morning chronotypes was also noted in the southern region. At the same time, among the northern young men, it was not revealed persons with a definitely morning chronotype, and among the Vladikavkaz students, it was not revealed persons with a definitely evening chronotype. In the seasonal winter/summer dynamics, an increase in the proportion of larks was found in all groups, less pronounced in young men. Among the students of Vladikavkaz in both seasons, young men predominated among the morning chronotypes, and the percentage of persons with an arrhythmic chronotype was higher among young women.
The values of the index of photoperiodic stability in general characterize the chronotype of the examined students as rigid, regardless of their territorial affiliation. At the same time, the average PSC values of larks in northerners corresponded to the lower limits of the values corresponding to natural physiological reactions to changes in the ratio of dark and light time of day, while in Vladikavkaz residents and in both groups of young women, indicators of "doves" approached them. The data obtained demonstrate the prevalence of social circumstances over natural time generators in the organization of life among students as a social group.
The question of the discomfort of the changed photoperiod, depending on the latitude of the place of stay, remains relevant, including in connection with the development of programs to preserve the health of student youth and the prevention of seasonal exacerbations of various diseases. The more significant the seasonal differences (wintersummer) in the intensity of such a generator of biorhythms as light, the less likely it is that throughout the entire calendar year the body will be able to organize its life activity according to its biological needs. In this case, in order to develop the optimal daily routine, a strictly regulated social rhythm should be a determining factor, taking into account biorhythmological preferences, but having the ability to compensate for the lack of a factora generator of time. This question is especially acute for higher education in high latitudes when organizing distance learning. Difficulties and mistakes in the selection of the wakefulness-sleep mode during the period when the student independently organizes his daily routine, often leads to the formation of desynchronosis of the physiological functions of the body. It was previously found that during distance learning, young women retained the classic type of dynamics of working capacity with a maximum in the daytime, while young men showed several peaks of activity, while the vital activity of junior students was less synchronized with the day-night cycle [24].
It is necessary to develop and implement methods aimed at organizing independent educational and other student activities, as well as leveling off-season differences in the magnitude of factorsgenerators of time. The most effective in this regard is the normalization of the light regime, with the help of additional lighting, spectral characteristics close to sunlight (winter season), and the reduction of "light pollution" through the use of thick curtains, blinds (summer).