Heart attacks that strike in the morning tend to cause more damage than those that happen at night, and scientists are uncovering why. Beyond daily swings in blood pressure and stress hormones, the body’s internal clock also shapes how the immune system responds during cardiac injury. That timing appears to influence how much heart tissue is ultimately damaged.
Key immune cells involved in inflammation become more aggressive during daytime hours. When a heart attack occurs in the morning, these cells are more likely to spread into surrounding tissue, increasing injury. At night, the same cells behave in a more controlled way, limiting collateral damage while still responding to the injury.
Animal and patient data suggest that heart attacks occurring when immune activity is lower are associated with less severe injury and better recovery of heart function. The findings highlight how circadian biology may affect cardiovascular outcomes and raise questions about whether future treatments could one day factor in the timing of immune responses during a heart attack.
SOURCE:
JAMA, February 6, 2026