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Introduction
Sedentary behaviour typically refers to activities of less than 1.5 metabolic equivalent units and can operationally be defined as sitting or lying down like watching television or working at a computer. 1 2 Sedentary behaviour is distinct from physical inactivity (ie, not meeting sufficient levels of moderate-to-vigorous physical activity). 3 Opportunities for sedentary behaviour are ubiquitous and US adults spend 50-60% of their day in sedentary pursuits. 4 Thus, even small adverse health effects of this behaviour may have profound effects at the population level. 5-7
Depression is a major mental disorder. By 2020, depression is predicted to be the second leading cause of disability, immediately behind cardiovascular diseases. 8 Observational studies have investigated the association between sedentary behaviour and depression, and the results were inconsistent. 9-28 A prospective study reported that long-time TV viewing (≥6 vs <2 h/day) was related to higher risk of depressive symptoms. 27 However, another prospective cohort study showed no significant association between prolonged computer use (>4 vs <2 h/day) and depression. 26 Other studies about the association between sedentary behaviour and the risk of depression have also provided conflicting results. 9-25 28
Therefore, we systematically conducted a meta-analysis by combining all available data of observational studies to: (1) derive an estimation of the association between sedentary behaviour and depression; (2) explore the potential between-study heterogeneity and (3) investigate the potential publication bias.
Materials and methods
Literature search strategy
We followed the PRISMA guidelines. 29 The available published studies reported in English or Chinese were identified by extended computer-based searches from PubMed, Web of Knowledge, Chinese National Knowledge Infrastructure and Wanfang databases up to 15 January 2014. Search terms included inactivity, computer use, sitting time, television viewing, screen time, media use, car driving and sedentary behaviour combined with depression and mental health. We also reviewed the reference lists from retrieved articles to identify additional studies not captured by our database search. The detailed steps of the literature search are shown in figure 1 .
Inclusion criteria
The inclusion criteria were as follows: (1) observational studies published as an original study to evaluate the association between sedentary behaviour and risk of depression, (2) relative risks (RRs) with 95% CIs (or data to calculate these) were provided. Depression was defined as...