A new wearable-based study reveals that not only how long you sleep, but how consistently you go to bed can have a big impact on your long-term heart health, especially if you’re not getting enough rest.
Study: Irregular sleep timing in midlife: association with serious adverse cardiac events and cardiovascular death over 10 years of follow-up. Image credit: Pixel-Shot/Shutterstock.com
Sleep timing reflects your body clock and daily habits that influence cardiovascular function. Disturbances in circadian regulation occur due to irregular behaviors and activities, such as shift work, late meal timing, and irregular sleep timing. In recent research, BMC cardiovascular disorders Over a 10-year follow-up, researchers found that irregular sleep timing was associated with an increased risk of major cardiovascular events and cardiovascular mortality (the composite outcome of MACE/CVD mortality).
Wearable to assess sleep timing fluctuations
Wearable sensors enable a more detailed sleep assessment by monitoring daily variations in multiple sleep parameters that determine sleep adequacy. Therefore, researchers developed device-based metrics such as interday stability and standard deviation (SD) of sleep timing. These are consistently correlated with cardiometabolic biomarkers in population studies.
For this study, the researchers decided to focus on the regularity of sleep timing, the consistency of participants’ bedtimes, wake-up times, and the midpoint of sleep over multiple days (between bedtime and wake-up time). The SD of each measurement was calculated for 7 consecutive days.
This observational study involved 3,231 participants (mostly women) from the 1966 Northern Finland birth cohort. These participants began wearing accelerometers from 2012 to 2014. They were followed in 2023 until a MACE event, death, or loss to follow-up.
Sleep timing regularity was assessed from accelerometer data processed by an existing algorithm. The accuracy of this method is within approximately 20 minutes compared to a sleep diary, within approximately 5 minutes compared to a smart ring assessment, and has been validated against the gold standard, polysomnography. Regularity was categorized by tertiles.
Regularity of sleep timing is associated with MACE
MACE includes heart attack, stroke, unstable angina, hospitalization for heart failure, or cardiovascular death (CVD). In this cohort, 4% (128 patients) experienced MACE during follow-up.
Being male, having a high body mass index (BMI), and being unemployed were associated with a higher risk of MACE. Those who experienced MACE had higher blood pressure, glycated hemoglobin, and LDL cholesterol than the group who did not experience MACE. Bedtimes, wake times, sleep midpoints, and median sleep duration were similar for both groups.
Nevertheless, the association between MACE and sleep timing irregularities was only observed among individuals who slept less than the median sleep duration within the group. The median is very similar to the recommended amount of healthy sleep, which is 7 hours and 56 minutes. In this subgroup, irregular bedtimes and sleep midpoints were independently associated with a doubled risk of MACE compared with regular bedtimes.
All associations persisted after adjusting for key cardiovascular risk factors, including gender, employment status, BMI, systolic blood pressure, LDL cholesterol, glycated hemoglobin, and physical activity, and additional variables were examined in supplementary and sensitivity analyses. Chronotype refers to an individual’s natural preferences for sleep and activity, such as morning, evening, or intermediate type.
Comparison with other studies
These findings are consistent with existing studies based on other indicators such as sleep regularity index and sleep onset variability. These reported similar associations between MACE or cardiovascular health and different aspects of irregular sleep.
However, no relationship was found with wake-up timing. A potential takeaway from this and similar studies is that wake time may not be as important for cardiovascular homeostasis in this cohort, but further studies are needed to confirm this.
Regular sleep is more important for health
The lack of such an association with irregular sleep when sleep duration exceeds the group median may suggest that sleep regularity plays an important role for cardiometabolic health alongside sleep duration. However, the study also notes that sufficient sleep may provide partial protection, making the results different from some previous studies. Irregular sleep timing can disrupt circadian rhythms and interfere with physiological recovery during sleep.
The association between increased MACE risk and unemployment highlights the need to consider mental health risk factors when examining the relationship between sleep and cardiovascular health. Such factors may increase the risk of depression and anxiety and further disrupt sleep patterns, but this pathway has not been directly tested in research.
strengths and limitations
This study is unique in that it uses accelerometer data to derive more reliable data on the regularity of sleep timing. The three aspects of sleep timing regularity were investigated separately and may provide actionable insights into behaviors that influence sleep health.
Accurately identified cardiovascular events from a unified database. The broad sample size and long follow-up period also support this finding. Additionally, reanalysis excluding participants who developed MACE early (within 2 years from baseline) reduces the risk of reverse causation.
Nevertheless, there are some limitations. The number of events is relatively modest and consistent with similar cohorts. A homogeneous population in terms of age and ethnicity. Also, the measurement period is rather short at 7 days (although this is supported by previous studies).
These findings highlight the importance of consistent sleep behaviors, especially regular bedtimes, as a potential goal for promoting health.
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Reference magazines:
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Nauha, L., Niemela, M., Azadifar, S. et al. (2026). Irregular sleep timing in midlife: Association with serious adverse cardiac events and cardiovascular mortality over 10 years of follow-up. BMC cardiovascular disorders. Toi: https://doi.org/10.1186/s12872-026-05762-4. https://link.springer.com/article/10.1186/s12872-026-05762-4
