Potential mechanisms
While the evidence examining lifestyle behaviours and mental health is growing, there is a notable gap exploring and revealing mechanisms underlying such associations [20]. Studies have typically examined associations, and then discussed possible explanations for findings as opposed to being driven by clear theoretical concepts from the outset. One possible explanation may be the complex range of mediators driving lifestyle behaviours, and the difficulties in extrapolating associated impact.
Although evidence of the independent effect of sedentary behaviour on health is emerging, it has been assumed that time spent sedentary is linked to physical activity [58]. Physical inactivity has been shown to interact with mental health [19, 59, 60], and exercise has been shown to have a positive effect in mental health treatment studies [61–63]. It is possible that the beneficial pathophysiological, social and general health effects of being active may be omitted when sedentary, which may have a negative impact on mental health. Adolescents who experience poorer mental health may lack motivation to be physically active and may turn to screen based activities requiring little effort as a coping mechanism, and therefore lose such protective effects of physical activity.
Although the relationship between mental health and sedentary behaviour remained significant after controlling for physical activity in six of the reviewed studies here [31, 43, 46, 49, 50, 54], only two used objectively measured physical activity [46, 54], and this relationship requires further investigation. Previous findings have indicated that the relationship between sedentary behaviour and physical activity among children and adolescents is negative, but small [64]. This suggests these behaviours do not directly displace one another, and support the examination of sedentary behaviour as a distinct behaviour. In addition, an adolescent reporting high levels of physical activity might be experiencing poor mental health alongside high levels of sedentary behaviour. The issue of whether physical activity moderates the assoiciation between sedentary behaviour and mental health is beyond the scope of this systematic review, but forms an important avenue for future research.
As with physical activity, there are known complex relationships between sedentary behaviour, overweight/obesity and other associated obesogenic risk behaviours, and this may have a significant negative impact upon adolescent mental health [20]. The relationship between unfavourable body composition [9], weight status [10] and increased levels of sedentary behaviour has been demonstrated among adolescents. Importantly, overweight and obese young people are known to experience particular mental health vulnerabilities including; weight-based teasing and bullying, stigmatisation, poor body image, and self-esteem, and this in turn is expected to impact on potential sedentary behaviour and mental health associations [65]. Weight status/body mass index was included as a covariate in nine of the reviewed studies [30, 32, 38, 39, 43, 44, 46, 50, 53]. It is reccomended that future research includes such weight-based measures as covariates in asessing the relationship between sedentary behaviour and mental health.
There is evidence supporting the role and impact of diet on mental health during adolescence [66]. The relationship between dietary and sedentary behaviours among this age group has also been previously reported [67]. Recently, an updated comprehensive systematic review [68] revealed that among young people, higher levels of sedentary behaviour (TV viewing in particular) is associated with less healthy dietary behaviours such as low fruit and vegetable consumption, and increased consumption of energy dense foods and sugar sweetened beverages. With known impact of nutritional deficiencies on mood [69] and the increased appetite for sweeter and processed foods when experiencing poorer mental health [70], it is possible that diet is implicated in the relationship between leisure screen time and mental health. Diet-related measures were included in four of the reviewed studies [32, 36, 39, 49], and as with weight-based measures, it is recommended that future research include diet as a covairate in assesssing the relationship between sedentary behaviour and mental health.
While outside the scope of this review, the type of activity being undertaken during screen time for leisure may have important implications in the context of adolescent mental health. It has been suggested that increasing time spent sedentary using screens may represent a population increased dependency on social media platforms for connectedness and interaction with peers [71]. Such experiences may hold protective benefits for mental health and an examination beyond time spent sedentary is needed to identify this potential mechanism. Similarly, it has been suggested that the Internet may offer increased access to health information, including mental health support [72, 73], and this relationship requires further investigation. The type of sedentary activity may be appropriate to examine in relation to mental health compared to objectively measured duration spent sedentary. In addition, screen use does not nessesarily equate to sedentary behaviour in metabolic terms and this further supports comprehensive examination of specific experiences in the context of mental health.