A New Clue to Why Heart Attacks Are Less Severe at Night

It’s been known for some time that myocardial infarctions, or heart attacks, tend to be more severe in the morning than they are at nighttime. Daily fluctuations in stress hormones and blood pressure are well documented and point to the important role circadian rhythm plays in cardiovascular health. But those aren’t the only factors.

Immune responses also follow the body’s natural biological clock. Research has shown that neutrophils—the most common type of white blood cell—often cause more damage to tissues at the site of inflammatory injuries, such as heart attacks, during the day than at night.

Neutrophils have been referred to by cardioimmunology experts as first responders for the speed at which they arrive to a trauma, but they’ve also been labeled as foot soldiers and demolition crews based on the havoc they wreak in the process of fighting off potential infections.

“They’re the first sentinel, but they come fully loaded,” said Douglas Mann, MD, the Ada L. Steininger Professor of Cardiology and professor of medicine, cell biology, and physiology at Washington University School of Medicine in St Louis. “They’re shooting at everything and dumping a lot of toxic granules on the environment. They are indiscriminate in terms of their ability to destroy, and they take out healthy cells in the process.”

Scientists have explored the connection between neutrophils and the severity of daytime heart attacks, but now, a new study may provide a novel clue into not just why neutrophils are diurnally aggressive but how, with some tweaks to their internal clocks, they may be modified to do less damage during noninfectious “sterile inflammation” while still eliminating pathogens.

What the researchers were able to show, Mann said, was a way “to train the foot soldiers” and minimize their collateral damage. “That’s a very big deal,” he said.