Irregular bedtimes double heart risk for those getting below 8 hours

Wearables to assess sleep timing variability

The use of wearable sensors enables a more granular sleep assessment by monitoring daily variability in multiple sleep parameters that determine sleep adequacy. Accordingly, researchers have developed device-based metrics, such as interdaily stability and the standard deviation (SD) of sleep timing. These consistently correlate with cardiometabolic biomarkers in population studies.

In this study, the researchers chose to focus on sleep timing regularity, consistency of a participant’s bedtime, wake-up time, and sleep midpoint (between bedtime and wake-up time) across multiple days. The SD of each measure was calculated for seven consecutive days.

This observational study included 3,231 participants, mostly women, from the Northern Finland Birth Cohort 1966. These participants began to wear accelerometers in 2012-14. They were followed up until 2023, a MACE event, death, or loss to follow-up.

Sleep timing regularity was assessed from accelerometer data processed by pre-existing algorithms. This method is accurate to within approximately 20 minutes compared with sleep diaries and about 5 minutes compared with smart ring assessments, which, in turn, have been validated against the gold standard, polysomnography. Regularity was classified by tertiles.

Sleep timing regularity linked to MACE

MACE comprises heart attacks, stroke, unstable angina, hospitalization for heart failure, or cardiovascular death (CVD). In this cohort, 4 % (128 individuals) experienced MACEs during follow-up.

All associations persisted after adjustment for key cardiovascular risk factors, including sex, employment status, BMI, systolic blood pressure, LDL cholesterol, glycated hemoglobin, and physical activity, with additional variables examined in supplementary and sensitivity analyses. Chronotype refers to the natural individual preference for sleep and activity, such as morning, evening, or intermediate type

Comparison with other studies

These findings agree with existing research based on other metrics, like the Sleep Regularity Index or sleep onset variability. These reported similar associations of MACE or cardiovascular health and various aspects of irregular sleep.

However, no relevant correlations with wake-up timing have been found. A potential takeaway from this and similar studies is that wake-up time may not be as critical for cardiovascular homeostasis in this cohort, though further research is needed to confirm this.

Sleep regularity is more important for health

The absence of such associations with irregular sleep when the duration exceeded the group median may suggest that sleep regularity plays an important role alongside sleep duration for cardiometabolic health; however, the study also notes that sufficient sleep may offer partial protection and that findings differ from some prior studies. Irregular sleep timing may disrupt circadian alignment, thereby hindering physiological recovery during sleep.

Strengths and limitations

This study is unique in using accelerometer data to derive more reliable data on sleep timing regularity. It separately examines three aspects of sleep timing regularity, offering potentially actionable insights into behaviors that affect sleep health.

It accurately identified cardiovascular events from a uniform database. Its broad sample demographics and prolonged follow-up period also strengthen the findings. In addition, the re-analysis excluding participants who developed early MACE (within two years of baseline) mitigates the risk of reverse causality.

Despite this, it has some limitations: a relatively modest number of events, consistent with similar cohorts; a homogeneous population with respect to age and ethnicity; and a somewhat short measurement time frame of seven days (though supported by earlier research).

Download your PDF copy by clicking here.

Journal reference:

• Nauha, L., Niemela, M., Azadifar, S., et al. (2026). Sleep timing irregularity in midlife: association with incident major adverse cardiac events and cardiovascular disease mortality over a 10-year follow-up. BMC Cardiovascular Disorders. DOI: https://doi.org/10.1186/s12872-026-05762-4. https://link.springer.com/article/10.1186/s12872-026-05762-4