Adolescents’ cognitive achievement not only strengthens individual human capital and future competitiveness in the labor market, but also enhances social innovation capacity and promotes economic growth (Cameron and Heckman 1998; Hunter 1986; McLeod and Kaiser 2004; Singh et al. 2019). It is necessary to clarify the mechanisms that contribute to the improvement of adolescents’ cognitive achievement. A growing number of policymakers and parents worldwide are paying attention to the impact of time use on well-being and mental health among adolescents (Borga 2019; Hunt and McKay 2015; Wight et al. 2009). If adolescents allocate a significant proportion of their discretionary time to positive and purposeful constructive activities, this will be beneficial to their healthy growth (Zill 1995). Some studies have analyzed adolescents’ time use. For instance, 81% of schoolchildren aged 11–17 years worldwide do not meet the World Health Organization’s recommendation of one hour of moderate to vigorous physical activity per day, and in China, the figure is 84.3% (Bull et al. 2020; Guthold et al. 2020). In the United States, about 45% of adolescents spend excessive time in front of electronic screens (Onyeaka et al. 2022). However, everyone’s time is limited, and there is competition between different aspects of time use. Considering the impact of only one aspect of time use on cognitive development may not be comprehensive, and all other aspects of time use need to be considered to understand the role of each aspect of time use on adolescents’ cognitive achievement. Thus, clarifying the relationship between time use and cognitive achievement will facilitate the effective utilization of time for adolescent development, and can provide a reference for targeted public health interventions. CIn this study, cognitive achievement is defined as a comprehensive measure of logical thinking and problem-solving abilities based on standardized cognition tests. Considering the fixed schedules of Chinese adolescents during school hours, researchers we investigate time use outside of school, including time spent on homework, playing sports, surfing the Internet, watching TV, and sleeping.

It is worth analyzing the impact of time use on cognitive achievement mechanisms, as this may shed light on the frequent negative emotions experienced by adolescents in real life (X. Liu et al. 2019b; Pekrun et al. 2002; Pekrun 2006). Depression symptoms are associated with adolescents’ time use and cognitive achievement, and may serve as mediating factors in explaining the relationship between the two. Unfortunately, depression symptoms among adolescents are becoming increasingly prevalent (Cao and Liu 2022; Gao et al. 2022; X. Liu et al. 2019a), with incidence rates rising from 5% at the start of adolescence to as high as 20% at the end of adolescence (Thapar et al. 2012). According to the World Health Organization, one in seven 10- to 19-year-olds worldwide suffers from mental health problems (WHO 2021). The COVID-19 pandemic has exacerbated this trend, with negative time use, such as decreased physical exercise and increased screen time (Neville et al. 2022), leading to cognitive decline and intensified depression symptoms in adolescents (Y. Liu et al. 2021; Xin et al. 2020). Despite this, there is limited research examining the mediating effect of depression symptoms on the relationship between time use and cognitive achievement, particularly in a Chinese context.

2. The Relationship between Time Use and Cognitive Achievement

Studies have explored the relationship between different aspects of time use and cognitive achievement. Most of the findings suggest that there is a negative relationship between time spent on the Internet and watching television and cognition. This may be because prolonged exposure to the Internet and television can cause changes in the structure and function of the brain, with potentially detrimental effects on higher-order cognition. Additionally, the variety and transience of stimuli on the Internet and television can easily lead to emotional fluctuations and distraction, resulting in cognitive decline (Kuss and Griffiths 2012; Mills 2014). A large-scale survey of American adolescents has revealed a negative correlation between screen time spent on television and social media and cognitive achievement (Walsh et al. 2020). A national survey of 17,076 American adolescents concluded that excessive screen time was detrimental to cognition, increasing the likelihood of cognitive difficulties, such as attention problems and memory impairment, by almost 1.3 times compared to those who did not overuse screens (Onyeaka et al. 2022). Jürges and Khanam (2021) found that educational activities were beneficial to cognitive achievement, while screen time, including watching TV, playing computer games, and social media use, increased internalization problems in Australian adolescents. Islam et al. (2020) discovered that addictive tendencies toward the Internet and electronic games were negatively associated with academic performance in 1704 Australian children aged 11–17 years, and recommended reducing Internet and gaming time through parental supervision and self-discipline. In addition, Internet use and gaming can also significantly disrupt adolescents’ sleep time (King et al. 2013) and increase the risk of mental disorders and dangerous behaviors (Green et al. 2020; Kliestik et al. 2020; Lăzăroiu et al. 2020; Rikkers et al. 2016). Previous literature has shown that homework time is an important factor in learning ability (Kumar and Choudhury 2021). Parents and teachers tend to believe that spending time of homework can benefit children’s cognitive development (Cooper et al. 2000). Some research has found a significant positive correlation between these two variables. A study based on 8–12-year-old adolescents in India showed that increasing homework time significantly improved the cognitive achievement of students who were lagging behind, suggesting that increasing homework time is one of the key ways to reduce the quality gap between private and public schools in India (Kumar and Choudhury 2021). Reinforcement theory proposes that continuous stimulation leads to changes in the brain’s cognitive and behavioral responses, increasing the likelihood of that behavior. Homework is an effective means of consolidating what students have learned, and spending time on homework helps to reinforce what they have learned in the classroom, thus having a positive effect on cognitive development (Skinner 1954). This has been demonstrated in studies of adolescents in Ethiopia, Vietnam, and India (Borga 2019). There is also evidence that the positive correlation between the two increases with grade level (Cooper and Valentine 2001; Cooper et al. 2006). However, there is currently no consensus on the effect of homework time on cognition, and some studies have found a negative correlation between the two. Chang et al. (2014) found that time spent on homework was negatively correlated with course outcomes based on a survey of 2342 students, possibly because smarter children require less time to complete their homework. Most studies suggest that there is a significant positive correlation between sleep time and cognitive development. Adequate sleep time can significantly improve cognitive achievement in adolescents, including performance in tasks related to executive function and multiple cognitive domains (Jones and Harrison 2001; Lo et al. 2016). A meta-analysis of 61 studies based on 71 populations showed that sleep restriction has a significant negative effect on executive function, long-term memory, and attention in cognitive performance (Lowe et al. 2017). A survey of 55 adolescents in the Netherlands found that longer sleep time significantly ameliorated visual spatial processing skills (Dewald-Kaufmann et al. 2013). The results from 8323 American adolescents showed that insufficient sleep (less than 9 h) has lasting negative effects on cognition and brain function (Yang et al. 2022). There are also studies suggesting that the correlation between the two is very weak (Astill et al. 2012) or even negative. For example, some surveys of elderly people in Japan and the Bronx found that respondents with longer sleep time had worse cognitive achievement (Kondo et al. 2021; Schmutte et al. 2007). Insufficient physical activity time presents a critical challenge for today’s adolescents worldwide. In total, 84.6% of Filipino adolescents do not meet the recommended daily minimum of one hour of physical exercise (Cagas et al. 2022). Only 27% of Thai adolescents achieve the daily threshold of 60 min for moderate-to-vigorous physical activity (Widyastari et al. 2022). Prior literature supports a significant positive association between physical activity time and cognitive achievement. Increasing physical activity time can enhance physical and mental functioning, leading to improved cognition (Hillman et al. 2008; Tomporowski et al. 2008). Children who participate in physical activity have larger basal ganglia and hippocampal brain volumes than inactive children, which is associated with superior cognitive control and memory performance (Chaddock-Heyman et al. 2014). A dearth of physical activity can lead to chronic diseases (such as diabetes and obesity) and mental health problems and impose a substantial economic burden on society (Colditz 1999; Hillman et al. 2008). Some studies have produced inconsistent conclusions, with a review of 58 intervention studies finding no conclusive evidence of the beneficial impact of physical activity time on cognitive achievement (Singh et al. 2019). This was also similar to the findings of a survey of Australian fifth–sixth grade primary school students (Dwyer et al. 1983). Previous literature also suggests possible gender and grade differences in time use (Lloyd et al. 2008). For instance, a survey of American adolescents showed that 8th graders spent more time watching TV and playing video games than 10th graders, and girls spent less time playing video games than boys (Tang and Patrick 2018). Twenge and Martin (2020) found significant gender differences in digital media use among American and British youth, with females spending more time on the Internet and social media, while males allocated more time to gaming and electronic devices. A study of 237 college students found that younger college students had shorter sleep time, while female college students had poorer sleep quality than males (Tsai and Li 2004). A study in Canada found that there were significant gender differences in weekend time use and that adolescents had less leisure time as their grade level increased (Hilbrecht et al. 2008). In conclusion, existing research lacks a comprehensive analysis of adolescents’ time use across different aspects. Time is a finite resource, and different aspects of time use compete with each other. A narrow focus on a solitary aspect of time use is not effective, and it is crucial to analyze the impact of multiple aspects on adolescents’ cognitive achievement. According to the characteristics and life trajectories of adolescents, their time use typically includes activities such as homework, sports, surfing the Internet, watching TV, and sleeping. It is therefore imperative to investigate the impacts of these common aspects of time use on cognitive achievement among adolescents and give full consideration to gender and grade differences. This will help parents and teachers to improve their parenting and teaching methods effectively.

3. The Mediating Role of Depression Symptoms

Previous literature has demonstrated a correlation between time use and cognitive achievement, yet little is known about the underlying mechanism. School achievement and psychological well-being are both crucial for adolescents, and a range of factors affect these outcomes, such as parental support (Moè et al. 2020), teacher enthusiasm (Moe et al. 2021), and also depression symptoms (Moè 2015). The present study focuses mostly on the latter variable by examining time use on the one hand, and cognitive achievement on the other. Depression symptoms may serve as the mediating factor that helps to explain this relationship. Control-value theory recognizes that emotions play a crucial role in cognitive processing and behavior. Positive emotions are often seen as a driving force for learning and development, resulting in improved learning efficiency and academic performance. Conversely, negative emotions have a detrimental effect, hindering behavior, learning outcomes, and personal growth (Pekrun et al. 2002; Pekrun 2006). Depression symptoms are the most common negative emotional expression among adolescents worldwide. The World Health Organization estimates that the burden of depression will exceed all other diseases by 2030 (Thapar et al. 2012). The negative impact of depression symptoms on adolescents is becoming increasingly apparent (Gao et al. 2020; X. Liu et al. 2022b), with the incidence among Chinese adolescents rising significantly to 57% following the outbreak of the COVID-19 pandemic (Chen et al. 2021). Unfortunately, few studies have focused on the mediating role of depression symptoms (Lim and Jeong 2022). A study of 562 Puerto Ricans over the age of 60 found that depression symptoms played a complete mediating role in the relationship between perceived daily discrimination and cognitive function (K. Wang et al. 2022). Relevant conclusions drawn from previous studies in other populations cannot be directly applied to the adolescent population, so it is of significance to examine the mediating effect of depression symptoms while taking into account the unique characteristics of adolescence. Research into the relationship between depression symptoms and cognition has accumulated a substantial body of literature, with the preponderance of evidence indicating that depression symptoms are significant predictors of cognitive decline (Conradi et al. 2011; Lee et al. 2012; Wagner et al. 2012), with 90% of depression patients experiencing impaired cognition (Hollon et al. 2006). A minority of studies have found little association between the two (Goulabchand et al. 2022; Grant et al. 2001). Studies investigating the impact of time allocation for tasks, such as homework and extracurricular activities, on the relationship between depression symptoms and cognition are relatively limited. A survey of Australian adolescents revealed that those with emotional issues or high levels of psychological distress spent the most time on the Internet or playing games (Rikkers et al. 2016). Neuroscientific evidence suggests that chronic sleep deprivation is a risk factor for the onset and progression of depression (Y.-Q. Wang et al. 2015). A study of 82 young adults found that Facebook use was associated with a decrease in subjective well-being. The more frequently people used Facebook, the lower their moment-to-moment experiences of happiness and life satisfaction tended to be (Kross et al. 2013). A review of adolescent mental health during the COVID-19 pandemic found that spending too much time on the Internet and social media can lead to increased symptoms of depression. This highlights how excessive screen time can be detrimental to the mental health of vulnerable adolescents (Oliveira et al. 2022). Although not all studies have confirmed this finding, a synthesis of 25 reviews reveals that the majority of research interprets the association between social media and mental health as weak or inconsistent (Valkenburg et al. 2022). A study of 79 suburban Florida high school students found that those with depression symptoms spent less time on homework and achieved lower average grades (Field et al. 2001). A significant positive association was observed between homework time and depression symptoms among Chinese primary school students in grades 2–6 (Xiao et al. 2022). However, the above literature only examined the relationship between individual time use and depression symptoms separately. Meanwhile, some individual studies have also explored the relationship between time use, cognition, and depression symptoms in specific domains. A survey of 3724 adolescents found that cognitive function moderated the effect of sleep time on depression symptoms (Zhou et al. 2022). A study of 280 retired individuals showed that self-reported depression symptoms were associated with slow stepping reaction time, a connection that could be explained by underlying cognitive impairment (Kvelde et al. 2010).