Косова, А. Л. Chronotype, sleep length, and school achievement of 11- to 23-year-old students in northern European Russia / Borisenkov M. F., Perminova E. V., Kosova A. L. // Chronobiol. Int. - 2010. - Vol. 27. - P. 1259-1270.

Students’ Chronotype and Sleep Length in the North, 1261 external factors on chronotype at northern latitudes is not sufficiently studied. In Norway, there is a phase delay of the sleep-wake rhythm during polar nights (Pallesen et al., 2001), and there is an elevated preva­ lence of late-types among inhabitants of the Komi Republic compared to those of countries in Central Europe (Borisenkov, 2010). Furthermore, desynchronization of the circadian system among residences of northern latitudes is common (Reinberg et al., 2007; Stokkan & Reiter, 1994). The sleep duration of children depends on cultural traditions and bio­ logical factors. There are essential differences in the sleep duration of resi­ dents of different countries (Jenni & O’Connor, 2005; Liu et al., 2005), and there is a steady reduction of sleep duration between age 1 and 20 yrs (Iglowstein et al., 2003; Russo et al., 2007). The chronotype of children depends mainly on biological factors. From age 10 to 20 yrs, there is phase delay of the sleep-wake rhythm (Carskadon et al., 1993; Giannotti et al., 2002; Roenneberg et al., 2004; Tonetti et al., 2008). The adaptation of late- chronotype children to the social environment is difficult, and as a result they may have lower school achievements (Carskadon et al., 1995), psycho­ logical problems, and deviant behavior (Gau et al., 2007; Yokomaku et al., 2008). One of the probable reasons for the negative effect of the school schedule on persons with late chronotype is an insufficient amount of sleep (Medeiros et al., 2001). Late-chronotype children are compelled to wake up early weekdays, but at the same time they go to bed late in accordance with their internal clocks. To compensate for the sleep debt, they wake up very late on weekends. As a result, there is a great instability of sleep time in late-chronotype children during the calendar week. This phenomenon has been termed “social jetlag” (Wittmann et al., 2006), and it can cause desyn­ chronization of circadian systems and affect school achievement. The purpose of the present study is to understand the relationship between chronotype and sleep length with place of residence and school achievements of children and college students living at northern latitudes. METHOD Geography, Climate, and Socioeconomic Conditions The investigation was carried out from February through May of 2009 in villages of the southern Komi Republic (KR), Syktyvkar, KR, Inta, KR, and Apatity in the Murmansk region (MR) of the Russian Federation (numbers 1-4, respectively, of Figure 1). Table 1 presents the characteristics of the settlements, i.e., geographical coordinates, dates of investigation, average year-round temperature, and photoperiod. The social and economic conditions, average educational level of residents, and average level of alcohol consumption in KR and MR are comparable (Kevesh et al., 2008). П І О Н Т Ѵ L * 1U m > i r r i r l i t i f l i r a i i i i С I ■ [ p 1