Adolescent mental health problems is a public health topic of global concern (Twenge et al., 2021; Arrivillaga et al., 2020). Globally, approximately 14.2 % of adolescents (aged 10–19) experiences a mental disorder, accounting for 13 % of the global burden of disease in this age group (Word Health Organization, 2021). The symptoms of depression and anxiety can first appear from childhood to adolescence and have varying degrees of severity (Philippot et al., 2022). In fact, approximately 10 % of adolescents have experienced subclinical depression (Bertha and Balázs, 2013), it is estimated that by 2030, depression alone will become the main cause of illness and disability among adolescents in the world, which lead to a huge social burden and long-term medical costs (Blakemore, 2019). Therefore, how to prevent and intervene in adolescent mental health problems has become an important issue that needs urgent attention.

In the digital age, information technology use has become an important determinant of adolescent mental health (WHO, 2021). China, especially in rural areas, the internet penetration rate of underage (aged 6–18) is 97.3 %, the average time of internet use on holidays is >5 h, 3.9 percentage points higher in rural areas than in towns (China Internet Network Information Center, 2022). However, with the penetration of the internet in rural areas, the negative effects of internet addiction, erosion of undesirable contents, and excessive consumption among rural adolescents have become increasingly prominent (Cyberspace Administration of China, 2018). Rural underage internet users lack parental supervision and restraint when using the internet, only 38.3 % of rural underage internet users indicated that their internet use time was often restricted by parents (CNNIC, 2022). Therefore, it is important to understand the relationship between the internet use time and mental health among rural adolescents. How does internet use time affect the mental health of rural adolescents? What are the intermediate channels? Is there heterogeneity in the relationship between internet use time and mental health? The answers to the above questions can provide important empirical references for the prevention and intervention of adolescent mental health problems. It can also provide important ideas for the promotion of adolescent mental health in rural areas of other developing countries.

Currently, the academic community has not reached a consensus on the relationship between internet use and adolescent mental health. Several studies have found that internet offers many benefits to adolescents in terms of delivering information, facilitating communication and providing resources (Rosenthal et al., 2016), appropriate use of the internet by adolescents can broaden their horizons, release stress, and relieve fatigue (Slater, 2007). For example, playing online games helps user to build a bridge between the virtual and real worlds, alleviate their stress, depression or anxiety, and increased mental flexibility, which has a positive effect on their physical and mental health (Granic et al., 2014). Additionally, the internet maintains and deepens interpersonal relationships (Gross et al., 2002). Using the internet not only helps adolescents to get guidance from their relatives or friends, but also increases adolescents' social participation, self-identity and well-being (Wellman, 2001).

However, many studies have found that internet use had a significant negative impact on adolescent mental health (Kraut et al., 1998; Kwak et al., 2022; Boer et al., 2021; Liu et al., 2023). Excessive internet use time can lead to internet addiction, reduce quality of life and sleep, and produce easily negative emotions such as anxiety, depression or stress among adolescents. One study conducted a longitudinal survey of adolescents in grades 7–11 in 31 schools in the Montreal district of Canada from 2012 to 2018, and found that for every hour increase in social media use time, adolescents experienced a 0.64 increase in depression score, the mechanism of which was that media use decreased adolescents' self-esteem (Boers et al., 2019). Furthermore, a series of studies based on Korean junior and senior high school students found that internet use time was significantly correlated with adolescent's mental health problems (Kwak et al., 2022), and that excessive use of internet not only worsened sleep satisfaction and depressive symptoms (Woo et al., 2021; Frison and Eggermont et al., 2017), but also affected their adult and future generations' lives (Choi et al., 2017).

In addition, some scholars have also found no significant longitudinal association between adolescent internet use duration and early depressive symptoms (Coyne et al., 2019; Vaterlaus et al., 2020) and that internet use was neither a cause nor an outcome of depressive symptoms (Takahashi and Adachi, 2022). Whether excessive internet use among adolescents has negative effects depends on individual psychological characteristics (Helsper and Smahel, 2019) and gender differences (Twenge and Martin, 2020). For example, one study found, based on an analysis of data from an 8-year longitudinal study, that the amount of time spent online by adolescents aged 13–20 years increased with age, but there was no significant association between the amount of time spent on social media and their depression (Coyne et al., 2019).

As stated above, the following shortcomings exist in the current studies: First, the majority of prior research focused on urban adolescents (e.g., Choi et al., 2017) or a specific regional (e.g., Frison and Eggermont, 2017; Kwak et al., 2022), but there is a relative lack of research on this topic in rural areas. Due to the urban-rural digital divide, in rural areas, which are more economically and educationally backward, internet use and rural adolescents' mental health issues should be concerned. Second, most existing studies use only cross-sectional data, without sufficient tests to identify the causality and mechanism (e.g., Barry et al., 2017; Wu et al., 2022). Finally, in terms of research methodology, existing studies have mostly conducted descriptive analysis, ANOVA or correlation analysis based on a small sample of convenience sampling (e.g., Arrivillaga et al., 2020) or survey data from a particular city (e.g., Barry et al., 2017; Coyne et al., 2019). Because of insufficient representativeness and randomness of the sample, accuracy of the results cannot be assured.

Based on this, the main contributions of this article are as follows: First, built upon displacement hypothesis analysis framework, we examined the causal relationship between internet use time and mental health among Chinese rural adolescents. Second, using CFPS 2018–2020 longitudinal data, a nationally representative survey with a large sample of data, fixed effects model and instrumental variable method were used to identify the causal relationship between Internet use and mental health. Finally, we further explored the content of internet use and mental health, and also examined the heterogeneity between the internet use time and mental health based on gender, education, and academic performance.