BACKGROUND: Beyond sleep duration and efficiency, the day-to-day regularity of bed- and wake-times is increasingly recognized as a distinct dimension of sleep health. Whether irregular schedules independently predict adverse health outcomes remains unclear.
OBJECTIVE: To synthesize evidence on the association between objectively or diary-quantified sleep regularity and mental, cardiometabolic, inflammatory, cognitive, lifestyle, sleep-disorder, and mortality outcomes in adults.
METHODS: A protocol registered with PROSPERO (CRD420251101936) guided the review. MEDLINE (PubMed), Cochrane Library, PsycINFO, and Google Scholar were searched from inception to July 1, 2025 using a comprehensive strategy incorporating validated regularity metrics (Sleep Regularity Index, interdaily stability, composite phase deviation, social jetlag, night-to-night standard deviations). After duplicate removal, 3140 unique records underwent title/abstract screening; 147 full texts were reviewed. Risk of bias was appraised with the Newcastle-Ottawa Scale. Heterogeneity of metrics and outcomes precluded meta-analysis; evidence was synthesized narratively following SWiM guidelines and graded with van Tulder's best-evidence taxonomy.
RESULTS: Fifty-nine primary studies met all criteria. Consistent, moderate-certainty evidence linked greater sleep-timing irregularity to higher depressive and anxiety symptoms, elevated body mass index, insulin resistance, hypertension, and incident cardiovascular events. Limited but directionally uniform evidence indicated higher inflammatory markers and poorer lifestyle indices in irregular sleepers. Prospective biobank data associated low Sleep Regularity Index with smaller hippocampal volume and a 26-53 % increase in dementia risk. Five low-bias cohorts showed 20-88 % higher all-cause mortality for the least regular sleepers, independent of sleep duration and quality. Mechanistic pathways implicated circadian misalignment, autonomic imbalance, and systemic inflammation.
CONCLUSIONS: Across diverse populations and measurement approaches, unstable sleep timing consistently portends poorer mental, metabolic, vascular, cognitive, and survival outcomes. Because sleep regularity is modifiable and objectively trackable with wearables, schedule stabilization merits elevation to a core public-health recommendation and a priority target for randomized intervention trials.
Circadian Biology