In our study, after controlling for potential confounding factors, lifestyle factors including Internet use, breakfast, exercise, sleep, and homework were significantly associated with depressive symptoms. Internet use and exercise were gender-related predictors of depressive symptoms, being significant only in girls. These findings implied that possibly complex mechanisms may exist between lifestyle and depressive symptoms in children and adolescents.
Skipping breakfast was the most meaningful impact factor of depressive symptoms in this study. The prevalence of depressive symptoms in children and adolescents increased with breakfast skipping frequency. Recent studies also reported a similar positive association between skipping breakfast and depressive symptoms [33, 34]. Moreover, Lars Lien found that the association was stronger for boys than girls, even after adjusting for possible confounding factors [35]. The observed gender disparities might be attributed to variations in breakfast quality, sample size, and environmental or cultural influences. It was well known that girls tended to show more body image distortions than boys in children and adolescents [36]. The heightened self-consciousness about their body shape in girls led them to intentionally reduce the quantity and quality of food [37]. In fact, consuming a nutritious breakfast containing grain-based and dairy products have shown to be associated with lower scores of depressive symptoms [38]. Differences in the results highlighted the need for further research in this area to shed light on the relationship between breakfast consumption and depressive symptoms, and why gender differences existed. Although there were no well-established mechanisms to explain why breakfast frequency may affect depression, having a healthy and regular breakfast might be considered an effective approach to prevent depression [39].
Internet use was pervasive among teenagers. This study demonstrated the risk of developing depressive symptoms increased with time spending more than 2 hours on the internet. The result was consistent with the research conducted by Mingli Liu et al [40]. Recently, in a longitudinal study of adolescents, researchers found that high use of mobile phones at baseline was correlated with a high level of depression at one year [41]. Excessive use of mobile phones replaced face-to-face contact would inevitably lead to subsequent social isolation and ultimately have depressive symptoms [42]. However, results from studies investigating the effect of internet use on mental health have been inconclusive [43]. Thom et al. found no relationship between internet use and depression in healthy adolescents [44]. The difference in findings could be illustrated by the fact that the authors' data were collected in 2009. In the past few years, due to the increasing popularity and affordability of handheld devices such as smartphones and tablets, internet use has become more common than ever before. Furthermore, the reliability and generality of the results were also limited by the small sample size. Given the differences in research results and quality, further research on the relationship and potential mechanisms of Internet use and depressive symptoms must be done.
Our study found that about a quarter of children and adolescents slept less than 6 hours per day on average, and 24.73% of them developed depressive symptoms. Due to the decreasing sleep time of children and adolescents worldwide [45], a series of negative consequences brought out by the lack of sleep have aroused widespread concern. 6 hours of sleep or less was defined as short sleep duration or deprivation in earlier studies [46] and would increase the risk of depressive symptoms significantly [47]. The possible mechanism for the relationship between short sleep duration and depressive symptoms was that short sleep duration led to alterations in the hypothalamic-pituitary-adrenal (HPA) axis [48], and sensitization of the HPA axis would contribute to the onset of depression [49]. Notably, a large cross-sectional study from Japan also found a U-shaped relationship between sleep duration and depressive symptoms in adolescents [50], suggesting that an optimal sleep duration against depressive symptoms may exist. In Japan, the lowest risk of depression was 7.5 to 8.5 hours of sleep for girls and 8. 5 to 9.5 hours of sleep for boys [51]. Similarly, in our study, the lowest risk of having depressive symptoms was found in both girls and boys who slept 8 hours to 10 hours.
Approximately 80% of adolescents worldwide had not met the minimum recommended guidelines for moderate-to-vigorous physical exercise [52]. Additionally, girls were less physically active than boys [53, 54]. Our study confirmed the gender differences in exercise levels and further found that engaging in exercise for 3 hours to 5 hours per day was a protective factor for depressive symptoms. There was also promising evidence that exercise could be utilized for the prevention and treatment of depressive symptoms [55]. A study found that any level of physical activity, even low levels such as walking less than 2.5 hours per week, could prevent future depression [56]. Furthermore, findings from some other studies suggested that increasing levels of physical activity may have beneficial effects on depressive symptoms in children and adolescents [57, 58]. Interestingly, in our study, girls who exercised for more than 5 hours had an increased risk of depressive symptoms, while there was no significant difference between exercise duration and depressive symptoms in boys. Besides, research had shown that physical activity, especially organized team sports, had a greater relief on depressive symptoms in girls [59] and was able to provide more mental health benefits [57]. This suggested that girls, in particular, should be encouraged to participate in appropriate levels of physical activity.
Spending excessive time on homework was also considered as a risk factor for depressive symptoms. When homework lasted longer than 1.5 hours, the likelihood of developing depressive symptoms increased with time. Asian countries placed a greater emphasis on academic achievement, resulting in children and adolescents in Asia facing higher academic pressure and exhibiting higher rates of depressive symptoms [60]. Particularly in China, children and adolescents encountered intense academic pressure [61] and were burdened with heavy homework loads [62]. The excessive academic load led to emotional changes in students, which in turn easily contributed to the development of depressive symptoms in the long term [63]. Previous studies have demonstrated that adolescents who spent longer time on homework tended to have later bedtimes and shorter nocturnal sleep [64–66], ultimately experiencing more school-related stress [67]. In this study, it was found that spending more than 1.5 hours and 2 hours on homework respectively posed a risk for depressive symptoms in girls and boys. Therefore, more attention should be paid to children and adolescents, especially girls, to avoid these negative outcomes. To alleviate the academic burden on these students, the Chinese government has implemented the “double reduction” policy since September 1st, 2021, which has reduced the prevalence of depressive symptoms to a certain extent [68].
In summary, this study provided additional evidence on the impact of gender differences and lifestyle factors on depression in Chinese children and adolescents. We not only observed a significant association between unhealthy lifestyles and depressive symptoms, but we also identified two gender-related lifestyle factors including Internet use and time spent on exercise. In light of the above findings and to prevent the occurrence of depressive symptoms, we recommend that schools, families, and public health institutions implement effective multidisciplinary health interventions to increase their awareness, especially among girls, about the adverse effects of unhealthy lifestyles and depressive symptoms.
Strengths and Limitations
Strengths of the study include a large number of participants recruited at random and assessed using standardized diagnostic and symptom rating tools. We considered the lifestyle as a whole to explore its impact on depressive symptoms. This provided new insights for the prevention of depressive symptoms and new evidence for the improvement of depressive symptoms through adopting a healthy lifestyle in the future.
However, this study has several limitations. Firstly, due to the nature of our variables, we did not further verify the interrelationships between the five factors we studied. Additionally, we did not explore how Internet use, breakfast, exercise, sleep, and homework combined to correlate with better mental health. Secondly, we only conducted a preliminary study on the five lifestyle factors in terms of time or frequency, without a comprehensive examination of detailed study on each factor to reflect the relationship comprehensively and systematically between different lifestyles and depressive symptoms. For instance, in addition to breakfast frequency, breakfast quality was also an important factor affecting depressive symptoms [38]. We plan to include more lifestyle factors that may affect depressive symptoms in our future studies and use more comprehensive questionnaires or scales to collect a broader range of information about participants' daily lifestyles, so as to further study the relationship between lifestyle and mental health.