Why are heart attacks more common in the morning?

Q. I heard that people are more likely to experience a heart attack in the morning than at other times of the day. Why?

A. Yes, it’s true that heart attacks are most likely to occur between 6 a.m. and noon, and the risk appears to be greatest within the first few hours of when a person wakes up. A number of physiological changes driven by the body’s internal clock, or circadian rhythm — combined with the rise in stress hormones that occurs with awakening — seem to underlie this phenomenon.

Most heart attacks happen when a plaque deposit inside one of the heart’s arteries ruptures (bursts open), leading to the formation of a clot that blocks blood flow to the heart muscle. Several factors help explain why this sequence of events is three times more likely to happen in the morning hours compared to the evening.

When you get up in the morning, levels of the stress hormone norepinephrine rise, increasing your blood pressure and priming your body to be active. Because you don’t eat or drink overnight, you are mildly dehydrated in the morning, which makes your blood slightly thicker. In addition, levels of a protein called PAI-1 and several other substances that cause blood to clot more easily peak in the early morning. Together, these conditions create a “perfect storm” that makes a heart attack more likely.

Recent research suggests that yet another circadian-driven factor might be at play: the body’s immune response. The most common type of white blood cells, neutrophils, travel through the bloodstream to help heal all types of injuries, including damage from heart attacks. However, neutrophils trigger an inflammatory response that can cause collateral damage, and this response seems to be greatest in the morning. People who are admitted to hospitals with a heart attack during daytime hours show higher neutrophil counts and more heart damage than those admitted at night, according to a 2025 analysis of more than 2,000 patients.

This study adds to the already recognized importance of the immune system’s role in causing plaque buildup and severe heart attacks. The higher number and heightened inflammatory effects of circulating neutrophils in the morning compared to evening means a greater risk of heart damage following plaque rupture and blood clot formation in coronary arteries.

The best defense is twofold: minimize plaque formation by lowering LDL (bad) cholesterol to your goal level, and dampen inflammation with lifestyle changes. That means regular exercise and a healthy diet, plus a statin drug if appropriate. Some people who have already had a heart attack and remain at high risk may also benefit from a medication such as colchicine, which can help reduce chronic inflammation.

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