According to World Health Organization's (WHO) report, from 2005 to 2015, the number of people suffering from depression have witnessed an increase of 18.4 %. Additionally, based on WHO data, in 2021, China is projected to experience 300 million patients and 54 million cases of depression (World Health Organization, 2021). Depression is expected to outnumber all other diseases in terms of illness burden by 2030 (Malhi and Mann, 2018). Furthermore, depression is the most prevalent mental disorder among adolescents, affecting 2.6 % of children and adolescents worldwide with a greater number of adolescent depression symptoms being reported globally (Polanczyk et al., 2015). Adolescent depression symptoms were more prevalently reported in the whole world. According to a meta-analysis, the incidence of depressive symptoms among children and adolescents in China ranged from 4.0 % to 41.0 %, with a total prevalence rate of 19.9 % (Rao et al., 2019). A recent study conducted among adolescents in India reported that 25 % had moderate to severe depression and 64 % suffered from mild depression (Moitra et al., 2020). In addition, reported prevalence rates for adolescents depression are 26.2 % in Malaysia (Ling et al., 2018), 17.2 % in Pakistan (Khalid et al., 2019), and 52.6 % in Iran (Babak et al., 2019). The high prevalence of depressive symptoms not only endangers adolescent's current physical and mental health, social adjustment, and quality of life, but also places a significant financial burden on society and families.

Sleep duration, a key health indicator (Marinelli et al., 2021), is critical for preserving an optimal adolescent health (Zhou et al., 2021). The National Sleep Foundation (NSF) recommends that teenagers aged between 14 and 17 years should get a daily sleep of 8 to 10 h (Hirshkowitz et al., 2015). However, a systematic review and meta-analysis focusing on healthy children and adolescents aged 3 to 18 years found that the pooled mean estimates of adolescent's night time sleep duration decreased as they age, with those aged 15 to 18 years only getting 7.4 h of sleep (Galland et al., 2018). A recent study found that middle and high school students in Korea slept for an average of 7.3 h per day, averaging 7.4 h for middle school students (aged 14–16) and 6.0 h for high school students (aged 17–19) (Statistics Korea, 2020). A white paper on the sleep index of China's children and adolescents in 2019 reported that 80 % of teenagers did not get the recommended 8 h of sleep (Society CSR, 2019). Meantime, the existing literature provides evidence on the differences in sleep duration on weekdays and weekends among adolescents. Wang et al. (Wang et al., 2017) found that the gap in sleep duration between weekdays and weekends increased from 1.3 h in grade 7 to 2.3 h in grade 12. Additionally, a longitudinal study of Chinese adolescents (grades 7–11) revealed the median weekend-weekday sleep difference was 2 h at baseline, 1 h at 1-year follow-up, and 2 h at 2-year follow-up (Liu et al., 2020). Thus, adolescents worldwide are experiencing serious and widespread public health issues due to inadequate sleep.

Existing researchers found that causes of depression symptoms in adolescents are complex. These include individual physiological and psychological factors, family environmental factors (Hou et al., 2018; Mian et al., 2019), individual behavioral characteristics (Lai et al., 2014; Sanchez-Villegas and Martínez-González, 2013), endocrine hormones (Cao et al., 2020; Wang, 2018), and so on. A study on a nationally representative adolescent population from the United States found that sleep patterns alterations were linked to higher probabilities of mood, anxiety, substance use, and behavioral disorders (Zhang et al., 2017). The sleep patterns comprised later evening bedtimes, shorter weeknight sleep duration, longer weekend bedtime delays, as well as both short and extensive episodes of weekend oversleeping. Nearly 40 % of Chinese adolescents with insufficient sleep reported depression or anxiety symptoms. While, sleep alterations can influence mood depression can present itself through sleep pattern disturbances. A systematic review and meta-analysis reported that depressed adolescents experienced more wakefulness in bed took longer to fall asleep and experienced more wakefulness during the night (Short et al., 2020). These results indicate an association between teenage mental health issues and sleep durayion (Society CSR, 2019).

But little is known about the relationship between lack of sleep and depressive symptoms. Further, the results of the existing studies are inconsistent. Few studies have found a positive correlation between depression symptoms and short sleep duration. A meta-analysis revealed that a short sleep duration was widely cited as a predictor of depression or anxiety symptoms in teenagers (Lovato and Gradisar, 2014). Many of the previous epidemiological longitudinal studies have found a link between sleep deprivation and adolescent depression (Roberts and Duong, 2014; Short and Louca, 2015). Tarokh (Tarokh et al., 2016) pointed out that the dorsal medial prefrontal cortex was disrupted due to short sleep duration, which had a detrimental impact on mental health. Furthermore, a study on Chinese adolescents found a U-shaped association between sleep duration and depressive symptoms, implying that both short and long sleep duration increased the incidence of depressive symptoms (Guo et al., 2020). However, Alfano (Alfano et al., 2009) found no conclusive link between teenage depressive symptoms and sleep disturbances. Another study using a nationally representative sample of adolescents showed no evidence of a relationship between sleep duration and regulating anxiety and depression (Li et al., 2018). Therefore, further research is required to confirm the association between depressive symptoms and sleep duration among teenagers. Further,it is significant to examine the effect of sleep duration separately for weekdays and weekends.

Thus, this study aims to explore the characteristics of depression symptoms and sleep time among adolescents in Taiyuan, North China. Further, it analyzes the interconnectedness between depressive symptoms and sleep duration separately for weekends and weekdays. Meantime, we undertook a subgroup analyses based on this interconnectedness. Thus, this study will serve as a reference point for prevention of adolescent depressive symptoms.