Non-linear Age-related Patterns of Sleep Architecture in First-episode Schizophrenia: A Restricted Cubic Spline Analysis

BACKGROUND: The dynamic relationship between age and sleep architecture in the early stages of schizophrenia remains poorly characterized. Traditional linear models may fail to capture the complex patterns of sleep stages across different age groups. This study aimed to examine the non-linear associations between age and sleep parameters in First-Episode Schizophrenia (FES) patients. METHODS: A total of 109 FES patients were recruited, and the objective sleep parameters were measured using Polysomnography (PSG). The Restricted Cubic Spline (RCS) analysis was employed to investigate the non-linear patterns of sleep stages across the observed age range (14-45 years), controlling for biological sex, Body Mass Index (BMI), and duration of illness. RESULTS: RCS analysis suggested statistically significant non-linear associations between age and selected sleep parameters. Specifically, the relationship between age and percentage of N3 sleep (N3P) demonstrated a U-shaped pattern (pnon-linearity =0.0007), with an estimated nadir at approximately 31.1 years, after which N3P appeared to stabilize or modestly increase. The slope was negative below approximately 31.1 years and positive above this age point. For the percentage of N2 sleep (N2P), the spline curve showed an inverted U-shaped cross-sectional association between age and N2P (pnon-linearity =0.03). N2P was highest among individuals in their early 30s, peaking at an inflection point of 31.3 years and relatively lower among both younger and older FES patients. No significant non-linear relationships were observed for Arousal Time (AT) and percentage of REM (REMP) (all p > 0.05). DISCUSSION: In FES patients, the relationship between age and sleep architecture was characterized by distinct non-linear age-related patterns rather than simple linear associations. The age of approximately 31 years appeared to be a critical inflection point for both N2 and N3 sleep stages in FES patients. CONCLUSION: These findings provide a more nuanced description of the sleep profile in early-stage SZ.