Test for nonlinear association between MEQ-5 scores and Depressive symptoms
In addition, we examined the non-linear dose-response relationship between chronotype (MEQ-5 score) and risk of depressive symptoms by restrictive cubic splines, as shown in Figure 2. After adjusting for age, sex, annual income, education, passive smoking, physical activity, living children, infertility treatment time, cause of infertility, frequency of insomnia, nocturnal wake frequency, daytime napping, social jetlag, and nighttime sleep duration, we found that the odds of depressive symptoms appeared to decrease with increasing MEQ-5 scores (P overall < 0.001); however, no nonlinear trend was observed (P non-linear = 0.526) (Figure 1 ).