Depression is one of the most challenging health problems in the world. The main symptoms of major depressive disorder (MDD) in children and adolescents include a marked decrease in interest in most activities, significant weight loss or gain, insomnia or hypersomnia, fatigue or loss of energy, feelings of worthlessness, suicidal ideation and so on (Bernaras et al., 2019). Approximately 280 million people worldwide suffer from depression, leading to reduced quality of life, disability, and premature death, contributing significantly to the global burden of diseases (Institute of Health Metrics and Evaluation, n.d.; World Health Organization, n.d.). The onset of depression typically occurs in adolescence or early adulthood (12–30 years) and has recently shown a sharply increase in prevalence (Crouse et al., 2021). Individuals who have the relevant depressive symptoms but do not meet the full criteria of MDD is known as sub-threshold depression. Although the prevalence of depressive symptoms in children and adolescents varies significantly across studies, a recent systematic review reported that almost 19.85% of children and adolescents suffered from depressive symptoms in China (Rao et al., 2019).

Insufficient sleep and skipping breakfast are becoming increasingly prevalent among children and adolescents, and both lifestyle behaviors are associated with the onset of depression. The potential mechanism of the association between insufficient sleep and depression might be due to the circadian dysrhythmias (Crouse et al., 2021). A survey including nearly 70,000 students reported that 81.2% of teenagers aged 13–17 years in China had a sleep duration <8 h/day (Deng et al., 2021). In 2021, the Ministry of Education issued the Notice on Further Strengthening the Sleep Management of Primary and Secondary School Students, which proposed that primary school students, junior high school students and high school students should sleep for 10, 9 and 8 h per day, respectively (Ministry of Education of the People’s Republic of China, 2021). Previous studies has revealed a U-shaped correlation between sleep duration and depression, and insufficient sleep was associated with increased risk of developing depression (Luo et al., 2018; Li et al., 2022; Sun et al., 2018).

A previous study reported that the skipping breakfast prevalence in Chinese students was approximately 10% (Chen et al., 2022). Breakfast skipping has been identified as an important determinant of unhealthy lifestyles, potentially serving as a proxy for other health-risk behaviors, such as alcohol consumption, tobacco use, physical inactivity, as well as depression and stress disorder (Peltzer and Pengpid, 2020). A potential link between skipping breakfast and depression might be that the conversion of carbohydrates to glucose after having breakfast contributes to the synthesis of serotonin, which can regulate the depressive symptoms (Ferrer-Cascales et al., 2018). The relationship between skipping breakfast and depression has been reported in several studies, while some others failed to find the association (Peltzer and Pengpid, 2020; Zahedi et al., 2020; Lee et al., 2017; Haghighatdoost et al., 2021).

Although the association of insufficient sleep and breakfast skipping with depression have been considered independently in several studies, the joint association of these two lifestyle behaviors with depression remains unclear (Luo et al., 2018; Li et al., 2022; Sun et al., 2018; Peltzer and Pengpid, 2020; Zahedi et al., 2020; Lee et al., 2017; Haghighatdoost et al., 2021). No previous study has investigated the joint association of skipping breakfast, insufficient sleep and depression in Chinese children and adolescents. Given the large number of youths in China suffer from depressive symptoms, especially with the influence of COVID-19 pandemic, the mental health of Chinese youths is facing a huge challenge. Identifying lifestyle behaviors that are linked to depressive symptoms, and intervening lifestyle behaviors early in the life stage can be of great significance to the mental health of youths. This study aims to examine the independent and joint associations of insufficient sleep and skipping breakfast with depressive symptoms in Chinese children and adolescents, and investigate whether these associations varied by age or sex.