This study investigated the variation in perceived tiredness among 11, 13 and 15-year-old 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, WHO-coordinated 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.