The period of junior high school age is a key development stage of adolescent mental health, during which mental maturity and physical and mental development imbalance may greatly increase the susceptibility of the students to depressed mood. Previous studies have shown that yin-deficiency constitution is a risk factor for depressed mood in adolescents, which has also been reported to be impacted by stressful life events and the time taken to fall asleep, but there is no systematic discussion about the mechanism of action in the relationships between the four factors.

To explore the relationship between yin-deficiency constitution and depressed mood in junior high school students, as well as the mediating effect of stressful life events and the time taken to fall asleep.

On March 2022, a convenient sample of junior high school students from a middle school in Meishan City, Sichuan Province were surveyed on their general data (gender, ethnicity, region of origin, grade), prevalence of yin-deficiency constitution assessed using the yin-deficiency Constitution subscale of the Traditional Chinese Medicine Constitution Scale for 7-14-year-olds, prevalence of stressful life events assessed using the Adolescent Self-Rating Life Events Checklist (ASLEC), and the time taken to fall asleep assessed using the items assessing how long it takes to fall asleep in the Munich Chronotype Questionnaire (MCTQ), and prevalence of depressed mood assessed using the Center for Epidemiologic Studies Depression (CES-D). The scores of yin-deficiency constitution, ASLEC and CES-D as well as the time taken to fall asleep were compared across the students by demographic characteristics. Spearman correlation analysis was used to explore the correlation between the variables. A multiple-mediator model was constructed to examine the mediating effect of stressful life events and the time taken to fall asleep on yin-deficiency constitution and depressed mood, and the significance of chain mediating effect was tested using bootstrap method.

The survey achieved a response rate of 98.1% (2 570/2 620) in the 2 620 surveyed students. The detection rate of depressed mood in the respondents was 32.8% (844/2 570). Female students had higher scores of yin-deficiency constitution, ASLEC and CES-D than male students (P<0.05). The yin-deficiency score, ASLEC score and CES-D score in grade 2 and grade 3 students was higher than that of grade 1 students (P<0.05). The ASLEC score in non-Han students was longer than that of Han students (P<0.05). The time taken to fall asleep in Han students was longer than that of non-Han students (P<0.05). The yin-deficiency constitution score, ASLEC score and CED-S score of students from rural areas was higher than that of those from towns (P<0.05). The average yin-deficiency constitution score, ASLEC score, the time taken to fall asleep and CES-D score in all respondents were 25.0 (15.0, 40.0) points, 36.0 (30.0, 44.0) points, 15.0 (8.6, 25.7) min and 11.0 (6.0, 18.0) points, respectively. The yin-deficiency constitution score was positively correlated with ASLEC score, the time taken to fall asleep and CES-D score (r_s=0.538, 0.151, 0.470, P<0.05). The ASLEC score was positively correlated with the time taken to fall asleep and CES-D score (r_s=0.224, 0.684, P<0.05). The time taken to fall asleep was positively correlated with CES-D score (r_s=0.289, P<0.05). Multiple mediator model-based analysis showed that yin-deficiency constitution positively predicted depressed mood (β=0.465, P<0.001). And it also positively predicted the level of stressful life events (β=0.511, P<0.001) and the time taken to fall asleep (β=0.066, P=0.004). The level of stressful life events positively predicted the time taken to fall asleep (β=0.228, P<0.001) and depressed mood (β=0.591, P<0.001). The time taken to fall asleep positively predicted depressed mood (β=0.139, P<0.001). The final results of Bootstrap test showed that significant effect of path dependence was found in the path of yin-deficiency constitution→stressful life events→the time taken to fall asleep→depressed mood〔mediating effect value =0.009, 95%CI (0.006, 0.012), P<0.001〕, indicating that the chain mediating effect of stressful life events and the time taken to fall asleep on yin-deficiency constitution and depressed mood was valid.

Yin-deficiency constitution can be used as a direct predictor of depressed mood in junior high school students, but it also can directly affect their depressed mood through its role played between stressful life events and the time taken to fall asleep as an independent mediator or a chain mediator. To reduce the incidence of depressed mood and improve mental health in this population, measures can be taken to improve the yin-deficiency constitution, improve the ability to cope with stressful life events and shorten the time taken to fall asleep.