Statistical analysis

Continuous and categorical variables were analyzed using the t test and chi-square test, respectively, performed with SPSS Statistics software (version 24.0; IBM Corp., Armonk, NY, USA).

A moderated moderation analysis was performed to assess the moderating role of brain responses to sleep-related pictures in the association between depressive symptoms and sleep disturbance in CID patients and GS, using the PROCESS macro (v3.0) for SPSS⁴⁶. The responses to sleep-related pictures and groups (CID vs. GS) were used as continuous and categorical moderators, respectively (Fig. 3). Depressive symptoms (BDI score) and sleep disturbance (sleep latency from the sleep diary) were included as independent and dependent variables, respectively. In between-group comparisons, behavioral and neuroimaging data were analyzed by adjusting for age, which could affect sleep disturbance. Conditional process analysis was performed using bootstrapping resampling (10,000 samples) to estimate 95% confidence intervals. Significant interactions were further tested using the Johnson–Neyman procedure implemented in PROCESS.

Moderated moderation model. A moderated moderation analysis was performed using the PROCESS macro (v3.0) for SPSS. In the model, group was entered as a moderator (Z) of the moderating effect of brain activity in response to sleep-related stimuli (W) on the association between sleep disturbances (dependent variable, Y) and depression (independent variable, X).