Positive mental health is central to adolescent well-being. The present study examines the prevalence of loneliness and positive mental health indicators (mental well-being and self-esteem) in four Nordic countries and associations between loneliness, mental well-being, and high self-esteem. This study is based on data from the Health Behaviour in School-aged Children (HBSC) study which was conducted in 2018 in Denmark, Finland, Iceland, and Sweden. Participants were 5883 15-year-old boys and girls. To examine the associations between loneliness, mental well-being, and self-esteem, structural equation modeling (SEM) was applied. In the comparison of Nordic countries, the prevalence of loneliness was highest among Finnish and Icelandic adolescents. High mental well-being and high self-esteem were most prevalent in Denmark and Sweden. In general, boys scored higher on positive mental health indicators and girls on loneliness. Loneliness was also a strong indicator of low mental well-being and low self-esteem in all Nordic countries. Loneliness is not only associated with mental health problems such as anxiety and depression, but it is also a risk factor for adolescents' positive mental health. Positive mental health is important for healthy maturation and there is a need to develop initiatives to reduce adolescent loneliness and so support positive development.

The positive mental health and well-being perspective represents innovative public health research of first-rank priority in Europe. Good mental health is both a state and a resource for everyday life. Hence, the concept often refers to a subjective feeling (hedonic component) as well as positive functioning (eudaimonic component). Different conceptualisations of mental health-related issues are a background to this paper, which gives a brief overview of three research issues in the Nordic countries. First, the development in the occurrences of adolescent mental health-related indicators such as life satisfaction, health, sleep, and school pressure. Second, review of Nordic methodological studies reporting on different mental health-related measures. Third, the selection of measures of positive mental health employed in the 2017–2018 Health Behaviour among School-aged Children (HBSC) data collection in the Nordic countries. Using the Nordic HBSC data for 2002–2014, it was found that symptom and problem-oriented analyses of mental health can improve our understanding of the challenges adolescents face. However, there is also a need to examine positive aspects of mental health in order to enhance our understanding of different mental health-related dimensions. New measures were included in the 2017–18 HBSC data collection in the Nordic countries, enabling researchers to answer different research questions including analysing factors mediating and moderating positive mental health among school-aged children. Extending the perspective from a symptom- and problem-oriented view to a more positive and asset-based perspective adds additional value to studies of mental health.

Associations between school-related stress and poor health, risk behaviours and low well-being are well documented. The aim of this paper was to estimate trends of perceived school stress experienced by boys and girls of different ages in the Nordic countries, and to describe trends in school stress between the Nordic countries. Nordic data from the Health Behaviour in School-aged Children study (HBSC) between 2002 and 2014 were used. The participants were aged 11–16 years. School stress was measured by a single item; "How pressured do you feel by schoolwork?" The participants answering "some" or "a lot" were categorised as reporting school stress.

Sweden, Norway and Denmark had lower prevalence of school stress compared to Finland and Iceland. There was an increase in Iceland, Finland and Denmark, whereas adolescents from Sweden showed a decreasing trend. In Norway, the level was stable. Boys showed a marginal decline in school stress whereas girls showed an increase, and school stress increased by age for the whole period. It is a challenge for the public education systems in the Nordic countries to develop policies and practices that provide children with the necessary tools to achieve knowledge and skill, and at the same minimise stress in school.

Sleep has been found to be an important factor in adolescents’ mental and physical health. The aim of the present study was to examine trends in sleep difficulty (i.e., difficulty falling asleep more often than once a week) in the Nordic countries among 11- to 15-year-olds. We analysed Nordic data from the Health Behaviour in School-aged Children study (HBSC), which is conducted every four years, looking specifically at trends in sleep difficulty over a 12-year period from 2002 to 2014. The participants were aged 11 to 15 years. The total number of participants across these years was 113,447. A large percentage (17% to 31% in 2014) of adolescents in the Nordic countries experience sleep difficulty, and these difficulties increased from 2002 to 2014 in Denmark, Finland, Iceland, and Sweden. Only in Norway was there a decrease, mainly due to a reduction in sleep difficulty among 11-year-old boys and girls from 2010 to 2014. Sleep difficulty among boys and girls are prevalent and generally on the rise in the Nordic countries with the exception of Norway.