Sleep regularity in adolescents: Associations with sleep, rest–activity indices, and prospective substance use and depression

Summary Sleep is important for adolescent development and has been linked to a variety of mental health difficulties. New research suggests that sleep irregularity may be more clinically relevant than relying on mean values. This study aimed to (1) quantify the relationship between the sleep regularity index (SRI), a novel parameter reflecting the probability that an individual will be in the same state [asleep or awake] 24 h apart, and other actigraphy‐derived sleep and rest–activity metrics, (2) evaluate concurrent and prospective associations between the SRI and mental health symptoms (depressive symptoms and substance use), and (3) test sex as a moderator. At baseline, a community‐based sample of adolescents ( n = 295, 56.27% female, 58.98% White, age = 15.44) wore an actiwatch for 1 week to measure rest–activity rhythms and sleep–wake cycles. Actigraphy data were used to derive sleep metrics, SRI, and nonparametric rest–activity indices. Adolescents also completed self‐report measures of substance use and depression at baseline and at 6‐month follow‐up. Partial correlations indicated that individuals with more sleep irregularity had less interdaily stability, less robust relative amplitude, lower sleep efficiency, shorter total sleep time, longer sleep onset latency, more social jetlag, and a later sleep midpoint. Regression analyses revealed that sleep irregularity was associated with a lifetime history of substance use, first‐onset substance use, and a prospective increase in depressive symptoms, even after accounting for sleep duration and subjective sleep disturbances. The sleep regularity index may be a potential modifiable risk factor for enhancing adolescent mental health.