Night Shift For The Heart: Cardiac Arrest After Dark Remains More Likely To Be Fatal

Every minute matters when someone’s heart suddenly stops. But a new study of nearly 875,000 patients reveals that when that minute happens, whether during the day or in the middle of the night, plays a stubbornly persistent role in whether someone lives or dies. Despite more than a decade of advances in emergency medicine, the gap between daytime and nighttime survival has not budged.

Researchers analyzing data from a massive national registry found that people who suffered cardiac arrest outside a hospital between 11:00 PM and 6:59 AM had about 16% lower adjusted odds of surviving with good brain function compared to those whose hearts stopped during the day. They also had about 15% lower adjusted odds of regaining a pulse at all. This nighttime disadvantage held steady from 2013 through 2024, showing no signs of improvement across an entire decade.

Published in JAMA Network Open, the study raises urgent questions about what exactly goes wrong after dark. Slower ambulance response times explain only a sliver of the problem. Why nighttime cases do worse remains only partly understood.

Nighttime Cases Show Worse Outcomes Across a Decade of Data

Researchers drew from a registry called CARES, short for the Cardiac Arrest Registry to Enhance Survival, which collects data from agencies covering roughly 186 million people, about 56% of the U.S. population. From January 2013 through December 2024, more than 1.1 million patients were identified. After excluding nursing home cases, missing records, and children, 874,415 adults remained. Typical patients were 64 years old, about 64% were male, and more than four out of five cardiac arrests happened at home.

Just over a quarter of all cases occurred during the nighttime window. While fewer cardiac arrests happened at night, those cases had meaningfully worse outcomes. During daytime hours, about 30.6% of patients regained a sustained pulse, and 9.3% survived to hospital discharge with good brain function. At nighttime, those figures dropped to 25.8% and 6.7%. After adjusting for age, sex, race, witnessed status, initial heart rhythm, bystander CPR, and suspected cause, nighttime was still independently linked to worse outcomes.

Survival odds started dropping around 8:00 PM and stayed low through 8:00 AM, worst between 1:00 AM and 6:00 AM, when odds of a good outcome were roughly 20% to 22% lower compared to noon. Tracking trends year by year, no meaningful narrowing of the gap appeared from 2013 to 2024.

Why Slower Ambulances Don’t Explain the Cardiac Arrest Survival Gap

One natural assumption is that nighttime cases do worse simply because ambulances take longer to arrive, or because patients who arrest at night are sicker. Researchers tested both.

Median ambulance response time at night was seven minutes versus about six during the day. A formal analysis found slower response time accounted for only about 12.6% of the total nighttime disadvantage. Getting there faster would help, but it would not come close to closing the gap.

To test whether sicker patients simply have more cardiac arrests at night, researchers focused on the most favorable conditions: arrests witnessed by a bystander with a heart rhythm that responds well to a defibrillator. Even there, the nighttime disadvantage was larger, not smaller. Nighttime patients had 26% lower adjusted odds of surviving with good brain function. Differences in patient health alone do not explain what’s happening.

Researchers also examined arrests witnessed by 911 responders already on scene, where response time is essentially zero. Nighttime survival was still lower, with 6% lower adjusted odds of a good outcome.

The Nighttime Penalty Persists Even After a Pulse Returns

Among patients who did regain a sustained pulse, nighttime patients were still less likely to ultimately survive. About 24.5% of nighttime patients survived to hospital discharge with good brain function, compared to 29.1% of daytime patients. After adjustments, nighttime was still tied to 7% lower odds of survival after the pulse returned.

Hospital-based care after a cardiac arrest may be part of the problem. Staffing and availability of advanced treatments are often reduced during early morning hours, which could worsen outcomes even after patients arrive at the emergency department alive.

What Researchers Still Cannot Explain

Much of the nighttime survival gap remains unexplained. The data could not capture CPR quality, how quickly bystanders recognize an arrest, access to public defibrillators, or paramedic medication timing. Even a technically “witnessed” nighttime arrest, say a family member jolted awake by a noise, may involve delayed recognition and a slower response.

Racial disparities in the data add another layer of concern. A higher proportion of nighttime cardiac arrests involved Black or African American patients (22.3% versus 20.2% during the day) and occurred at home (88.8% versus 81.0%), raising questions about overlapping inequities that may compound the problem.

More than 870,000 cases spanning a dozen years make the pattern hard to dismiss. Until researchers can determine what is driving this gap, whether it involves bystander behavior, hospital staffing, differences in what happens during sleep, or something else entirely, the time on the clock will remain one of the least understood factors in whether someone survives a cardiac arrest.

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Disclaimer: This study was observational in design, meaning it identified associations between nighttime cardiac arrest response and survival outcomes but cannot establish that nighttime conditions directly cause worse survival. Individual circumstances vary, and this article is not intended as medical advice.

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