SUMMARYThis study investigated the variation in perceived tiredness among 11, 13 and 15-yearold Finnish adolescents (n=4187). Additionally interrelationships between sleep habits, use of psychoactive substances (alcohol, tobacco and coffee) and perceived tiredness among 15-year-olds were examined. This study is part of an international, WHOcoordinated survey of school children's health and lifestyle (the HBSC Study). In Finland, research data represented the whole country. The data were collected in March-May 1994. Pupils responded anonymously to a standardized questionnaire during a class period. Subjective tiredness was very common and increased with age among adolescents. Perceived tiredness on at least four school mornings a week increased from 24 to 35% among boys and from 16 to 34% among girls. Feeling tired more often than once a week increased from 20 to 37% in girls and from 24 to 50% in boys. Structural equation models showed that interrelationships between three factors -sleep habits, use of psychoactive substances and perceived tiredness -were statistically significant. For these three factors the two remaining factors explained 24% of the variance of perceived tiredness among boys and 20% among girls, and the two remaining factors explained 42% (16%) of the variation in sleep habits. For the variance of the use of psychoactive substances sleep habits and perceived tiredness explained 26% (12%). Subjective tiredness is strongly age related; this together with the use of psychoactive substances and sleep habits regulate adolescents' daily life and well-being.

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