Circadian Rhythms May Heighten Nighttime Cardiovascular Risks in People With Obstructive Sleep Apnea

New research is shedding light on why people with obstructive sleep apnea (OSA) appear to face a higher risk of cardiovascular events during the night, unlike the general population whose heart attacks typically cluster in the early morning hours after waking. Scientists at Oregon Health & Science University (OHSU) have identified a clear pattern: the body’s internal circadian clock seems to worsen blood vessel function overnight in people with untreated OSA. This natural time-of-day effect may create a window of increased vulnerability in the middle of the night, especially around 3 a.m., when vascular function appears most impaired.

Published in the Journal of the American Heart Association (2025), the study offers uniquely controlled evidence that the circadian system itself—not just sleep quality, activity levels, or breathing interruptions—plays a direct role in shaping cardiovascular risk in people with sleep apnea. The findings emphasize the importance of early diagnosis, proper treatment, and potentially new avenues involving timed medication for those who cannot tolerate conventional therapies.

What the Study Looked At

To isolate the effect of the internal clock from outside influences, researchers used a highly controlled laboratory setup at OHSU’s Marquam Hill campus. They recruited 12 adults with untreated obstructive sleep apnea, including both men and women, with an average age of about 51 years. Each participant stayed in the sleep lab for five consecutive days, during which all major external factors—such as exercise, meal timing, and sleep schedules—were held constant.

To track changes across the full 24-hour circadian cycle, participants underwent 10 recurring sleep cycles, each lasting five hours and 20 minutes. This allowed researchers to observe how vascular function changed at different internal biological times, regardless of whether participants were asleep or awake.

Blood vessel health was monitored using noninvasive ultrasound, specifically measuring flow-mediated dilation (FMD) of the brachial artery. This test assesses how well blood vessels expand in response to increased blood flow, which is a key indicator of endothelial function—and a strong predictor of cardiovascular risk.

What the Researchers Found

Across all participants, the team observed consistent circadian rhythms in vascular function. The most important finding: even when participants were awake, blood vessel function was at its lowest around 3 a.m. This time point represents a strong circadian phase where endothelial function naturally dips. But in people with obstructive sleep apnea, this dip was large enough to be considered clinically meaningful.

Researchers noted that this is the first controlled study showing that the circadian system directly worsens vascular function in people with untreated OSA. This impairment occurred independently of sleep quality, meaning the biological clock alone is influencing cardiovascular risk overnight.

Interestingly, the baseline diameter of the artery—measured when the vessels were at rest—did not show circadian variation. This means the effect was specific to how blood vessels responded, not their physical size.

The study’s findings provide a plausible explanation for why people with sleep apnea tend to experience cardiovascular events—such as heart attacks, strokes, and sudden cardiac complications—more frequently during the night.

Why These Results Matter

Obstructive sleep apnea has long been linked with elevated risks of hypertension, heart disease, stroke, and metabolic disorders. The new study adds an important layer: the body’s circadian biology may amplify these risks during certain hours.

In the general population, most cardiovascular events occur in the first few hours after waking, a period associated with natural morning increases in blood pressure, heart rate, and clotting activity. However, people with untreated OSA tend to have events throughout the night, which previously lacked a clear explanation.

This research suggests a mechanism: an internally driven nighttime drop in vascular function, making blood vessels less capable of expanding and responding to stress. When combined with the repeated drops in oxygen (hypoxia), surges of sympathetic activity, and inflammation characteristic of sleep apnea, the risk becomes even greater.

This also raises the potential for timed interventions—such as taking certain medications or antioxidants right before bed—to help improve overnight vascular health. However, this idea still needs testing in future trials.

Current Treatment Landscape for Sleep Apnea

The go-to treatment for obstructive sleep apnea is Continuous Positive Airway Pressure (CPAP) therapy. This device keeps the airway open during sleep using mild air pressure. CPAP is highly effective when used consistently, but many patients struggle with comfort, noise, or dryness, leading to low long-term adherence.

For people who cannot tolerate CPAP, options include:

Oral mandibular advancement devices
Lifestyle changes, such as weight loss, reduced alcohol use, and side-sleeping
Upper airway surgery, in select cases
Hypoglossal nerve stimulation, a newer implantable treatment

The new study highlights an urgent need to support vascular health overnight in OSA patients—especially those who remain untreated or cannot use CPAP effectively.

Understanding Circadian Rhythms and Heart Health

Circadian rhythms are 24-hour cycles controlled by the body’s internal clock, influencing sleep, hormone production, metabolism, and cardiovascular function. In healthy individuals, the circadian system already causes predictable changes in vascular function, autonomic activity, and blood pressure. But when a disorder like sleep apnea disrupts normal oxygen and chemical signaling during sleep, the circadian system may amplify vulnerabilities rather than protect against them.

Research outside of this specific study has shown:

Endothelial function naturally dips at night and early morning, even in healthy people.
Shift workers, whose circadian rhythms are frequently disrupted, experience higher rates of cardiovascular disease.
Chronic circadian misalignment can impair glucose regulation, blood pressure control, and inflammatory responses.

The new findings suggest that OSA interacts with these natural rhythms, making certain hours especially high-risk.

What Comes Next in This Line of Research

The study authors emphasize the need for:

Larger sample sizes to confirm these findings
Studies evaluating whether circadian-timed therapy can reduce real cardiovascular events
Investigations into whether CPAP normalizes the circadian disruption
A better understanding of how molecular clocks in vascular tissue are altered by sleep apnea

This research opens the door to improving care for millions of people living with OSA, many of whom are unaware they even have the condition.

The Bottom Line

This study provides important evidence that the body’s internal clock plays a significant role in determining cardiovascular vulnerability in people with untreated obstructive sleep apnea. By showing that blood vessel function is most impaired around 3 a.m., the findings help explain why nighttime heart events are more frequent in this population and highlight the importance of managing OSA proactively.

Even though the study was small, its controlled design offers a strong starting point for future research, especially into potential timed treatments that could help protect cardiovascular health during the night.

Research Reference:
Endogenous Circadian System Attenuates Nighttime Vascular Endothelial Function in People With Untreated Obstructive Sleep Apnea (Journal of the American Heart Association, 2025)
https://www.ahajournals.org/doi/10.1161/JAHA.125.043596