Irregular Sleep Schedules Rival Sleep Duration as a Cardiometabolic Risk Factor

Sleep irregularity — defined as day-to-day variability in sleep timing, duration, or both — has grown increasingly common in modern 24/7 societies driven by shift work, social obligations, and pervasive artificial light. Despite this prevalence, it has historically received less research attention than sleep duration or sleep disorders. This 2025 narrative review in Circulation Research by Tianyi Huang (National Institute on Aging) synthesizes the current epidemiologic, clinical, and mechanistic literature to establish sleep irregularity as an independent and potentially underappreciated cardiometabolic risk factor.

The review prioritizes large prospective cohort studies over cross-sectional designs to better establish temporal relationships. Evidence consistently links irregular sleep to elevated risks of obesity, metabolic syndrome, type 2 diabetes, hypertension, coronary heart disease, stroke, and all-cause and cardiovascular mortality. A particularly striking finding is that, in several studies, sleep irregularity explains cardiometabolic risk over and above average sleep duration — suggesting it captures a distinct dimension of sleep health that standard duration-focused metrics miss.

Huang proposes multiple interacting biological and behavioral pathways. Circadian misalignment caused by shifting sleep schedules desynchronizes peripheral clocks in metabolic tissues, impairing glucose regulation, lipid metabolism, and blood pressure rhythms. Irregular sleep also disrupts hormonal axes, including elevated cortisol, altered leptin/ghrelin balance promoting hyperphagia, and reduced insulin sensitivity. Behaviorally, erratic schedules are associated with poorer diet quality, reduced physical activity, and greater sedentary time. Psychologically, sleep irregularity is linked to mood disturbances and chronic stress, which independently promote cardiometabolic dysfunction.

The review acknowledges important limitations. First, there is no consensus on how best to measure sleep irregularity — metrics range from standard deviation of sleep duration or midpoint, to social jetlag, to composite irregularity indices — making cross-study comparisons difficult. Second, intervention studies that test whether regularizing sleep improves cardiometabolic outcomes are scarce, limiting causal inference. Third, confounding by unmeasured socioeconomic, occupational, or psychiatric factors remains a concern in observational data. Fourth, the directionality of some associations is uncertain, as cardiometabolic disease itself may worsen sleep regularity.

The clinical and public health implications are significant. If sleep irregularity is confirmed as a modifiable risk factor, sleep hygiene interventions targeting schedule consistency — not just total sleep time — could become a meaningful component of cardiometabolic disease prevention. Huang calls for standardized measurement frameworks, larger intervention trials, and deeper mechanistic studies to close these gaps and ultimately reduce the global burden of cardiometabolic disease.