The old adage “early to bed, early to rise makes a man healthy, wealthy, and wise” has shaped sleep advice for centuries. But according to new research published in Sleep Health, both early birds and night owls face a substantially higher risk of heart attack compared to people with an intermediate chronotype. The conventional belief that morning preference is inherently cardioprotective may be overly simplistic, the authors argue.
Researchers from the Fourth Military Medical University in Xi’an, China, analyzed data from 4,578 participants in the Sleep Heart Health Study (SHHS), a multi-center U.S. cohort that includes data from Framingham, ARIC, and other well-known sleep studies. Participants had a mean age of 63.4 years, 54.9 percent were female, and 86.8 percent were White. Over an average follow-up of 10.6 years, 291 myocardial infarctions occurred (6.4 percent of the sample).
The study used the Munich ChronoType Questionnaire (MCTQ) to derive a rigorous, objective measure of chronotype called MSFsc (mid-sleep time on free days, corrected for sleep debt). MSFsc correlates strongly with the gold-standard biomarker of circadian timing, dim-light melatonin onset, making it more precise than single-question “morningness” assessments used in some prior research.
Using MSFsc cutoffs established in the literature, the team classified participants as morning types (MSFsc earlier than 2:15 a.m., 23.9 percent), intermediate types (MSFsc between 2:15 and 3:30 a.m., 56.8 percent), and evening types (MSFsc later than 3:30 a.m., 19.3 percent). These labels translated to real-world sleep schedules: morning types typically slept from 10:03 p.m. to 5:21 a.m., intermediates from 11:10 p.m. to 6:37 a.m., and evening types from 12:29 a.m. to 8:05 a.m.
The U-Shaped Risk Curve
When the researchers modeled heart attack risk using the intermediate group as the reference, both extremes showed elevated hazard:
- Morning types: 50 percent higher risk of MI (HR 1.50, 95% CI 1.10-2.05, p=0.01)
- Evening types: 40 percent higher risk of MI (HR 1.40, 95% CI 1.00-1.95, p=0.04)
These results come from the fully adjusted Model B, which accounted for age, sex, race, hypertension, diabetes, smoking, body mass index, alcohol use, cholesterol, HDL, triglycerides, sleep duration, apnea-hypopnea index (AHI), and time spent with oxygen saturation below 90 percent. The findings were robust to sensitivity analyses including exclusion of the first six months of follow-up, and the proportional hazards assumption was not violated (p=0.22).
Sleep Apnea Amplifies the Risk
A striking finding emerged when the researchers stratified by obstructive sleep apnea (OSA). Among participants without significant OSA (AHI <5), neither morning nor evening chronotype carried a statistically significant elevation in heart attack risk. But among those with OSA (AHI 5 or higher), the pattern was dramatic:
- Morning type with OSA: 85 percent higher MI risk (HR 1.85, 1.25-2.73, p=0.002)
- Evening type with OSA: 82 percent higher MI risk (HR 1.82, 1.18-2.81, p=0.007)
The interaction between chronotype and OSA severity reached statistical significance in a four-category analysis (p=0.01), suggesting that circadian misalignment and sleep-disordered breathing may synergistically stress the cardiovascular system. Baseline data showed that morning types actually had the highest AHI scores, while intermediate types had the lowest MI incidence at entry.
What This Means
The study shifts the conversation from “morning is good, night owl is bad” toward a more nuanced view: it may be the extremes in either direction that matter. The intermediate chronotype, representing roughly 57 percent of the population, appears to sit at the bottom of a U-shaped risk curve.
The biological mechanisms are plausible. Circadian timing systems regulate blood pressure, heart rate, autonomic nervous system tone, endothelial function, and platelet activity. When an individual’s internal clock is consistently misaligned with their external schedule, these regulatory processes may be chronically disrupted, gradually increasing cardiovascular risk.
The findings align with UK Biobank research that reported a similar U-shaped relationship using single-item morningness-eveningness questions. The current study strengthens that evidence by employing the more precise MSFsc measurement.
Limitations
The SHHS cohort is older and predominantly White, limiting generalizability to younger or more diverse populations. The researchers lacked data on shift work, family history of heart disease, diet, physical activity, geographic latitude, or CPAP adherence. As an observational study, no causal conclusions can be drawn. The OSA interaction analyses were exploratory and were not powered a priori. Effect sizes are modest, with hazard ratios in the 1.4 to 1.5 range.
Bottom Line
Both morning and evening chronotypes carry a moderately elevated risk of myocardial infarction compared to intermediate types. The risk is amplified substantially in people with obstructive sleep apnea. While the study cannot prove causation, it suggests that the healthiest sleep schedule may not be the earliest or the latest, but the one that avoids either extreme.
Source: Zhu Y, Liu J, Zhao Y, et al. Association between chronotype and myocardial infarction in the Sleep Heart Health Study. Sleep Health. 2026. DOI: 10.1016/j.sleh.2026.05.010. PMID: 42498583.