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.

1266 M. F. Borisenkov et al. TABLE 3 ANCOVA of sleep length and chronotype of schoolchildren and college students depending on age, sex, geographical coordinates, types of settlements, and achievements Source0 Dependent variable d f F P n2 Age Sleep length 12 10.50 .00001 0.104 MSFsc 12 10.11 .00001 0.100 Sex Sleep length 1 0.V3 .39 0.001 MSFsc 1 21.02 .00001 0.019 Place of residence Sleep length 3 14.26 .00001 0.038 MSFsc 3 V6.56 .00001 0.173 Latitude Sleep duration 1 8.64 .003 0.008 MSFsc 1 135.55 .00001 0.110 Longitude Sleep length 1 0.32 .57 0.0003 MSFsc 1 215.VV .00001 0.164 Type of settlements Sleep length 1 41.44 .00001 0.036 MSFsc 1 19.9V .00001 0.018 Achievements Sleep length 3 4.V0 .003 0.013 MSFsc 3 8.33 .00001 0.022 aA one-way ANCOVA was performed using each factor (age, sex, place of residing, and achievements) one after another as independent variables while the rest were used as covariates. age-dependent reduction of sleep duration is in agreement with pub­ lished literature (Jenni & O’Connor, 2005), and it is defined by biological factors. The sleep duration of Russian 11- to 16-yr-old teenagers is much shorter than that of their Italian (Russo et al., 2007), Swiss (Iglowstein et al., 2003), Canadian (Laberge et al., 2001), American (Crowley et al., 2007), Indian (Gupta et al., 2008), and Chinese (Yu et al., 2007) counter­ parts. Our findings for the average sleep duration in children are consist­ ent with those of South Korea (Yang et al., 2005) and Saudi Arabia (BaHammam et al., 2006). The average sleep duration of Russian 17- to 23-yr-old teenagers does not differ from that of other countries (Steptoe et al., 2006). We hypothesize that the mean sleep duration of children and teenagers depends to a lesser degree on climatic factors and to a greater degree on cultural traditions (Jenni & O’Connor, 2005). The analysis of covariance showed there is significantly longer sleep duration in rural than urban-dwelling children (Table 3). The rural-urban differ­ ences in sleep duration appear to be explained by differences in socioeco­ nomic conditions (Ursin et al., 2005). Chronotype The chronotype of teenagers depends on their age and sex. We found a steady displacement to a late chronotype from ages 11 to 21 yrs. The phase delay of the sleep-wake rhythm in boys was stronger than in girls. These findings are consisted with those reported by other authors (Carskadon et al., 1993; Frey et al., 2009; Roenneberg et al., 2004; Tonetti et al., 2008). Hence, it was proposed that age- and sex-dependent