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, 1267 changes in chronotype are mainly determined by biological factors (Hagenauer et al., 2009; Roenneberg et al., 2004). We noted a stronger displacement to late chronotype in our 11- to 20-yr-old children than their counterparts from Central Europe (Table 2; Roenneberg et al., 2004). Most likely, these distinctions are the result of latitude of residence. ANCOVA (Table 3) confirms this. Children’s chron­ otype to a greater degree depends on the geographical coordinates of the settlements, as documented by the significant phase delay of the sleep- wake rhythm in the South-North and East-West directions. These results are in agreement with our previous finding (Borisenkov, 2010), showing the percentage of late chronotype is significantly higher among inhabi­ tants of KR than inhabitants of Central Europe. The greatest shift to late chronotype occurred among school children and college students resid­ ing in the arctic city of Apatity (Table 3). Such a shift is most likely caused by the extreme seasonal changes in photoperiod. Roenneberg et al. (2007) showed the time of sunrise is the external synchronizer of the human circadian system that influences the chronotype. Decrease in the strength of this synchronizer leads to a phase delay of the circadian system and, consequently, increase in the percentage of late-type subjects. It is obvious that in the Arctic region, the time of sunrise is a weak syn­ chronizer for the human circadian system. Moreover, Apatity is located on the western border of the time zone. As mentioned previously (Randier, 2008a, 2008b; Roenneberg et al., 2007), there is a shift to late chronotype in the East-West direction within one time zone caused by a divergence between social and solar times. Social time is identical for all inhabitants of a given time zone, whereas time of sunrise progressively increases from East to West. The standard time zone stretching for 15° of longitude is enough to influence chronotype (Roenneberg et al., 2007). Our research was carried out in the settlements located on the edges of an unusually wide time zone stretching 30° East of longitude. Similarly, significant correlations between chronotype and latitude in Central Europe have been described (Randier, 2008b). In contrast, the author noted an increase in the percentage of teenagers with morning chrono­ type in the South-North direction. This discrepancy might be explained by differences in latitude of residence of the investigated teenagers; the teenagers in Randier’s study lived at a moderate latitude where seasonal changes in photoperiod are less extreme than in the North. School Achievement There was a clear dependence of school achievement on chronotype (Table 3). Late-chronotype school children and college students had poorer achievement than intermediate and early-type students. Similar findings were reported by Randier and Freeh (2006). Sleep length also influenced Й I P H T I l l H W ) Г тп г і і і і f I i i i i i i i f u l i