Researchers have found that the internal body clock may contribute to the morning peak in heart attacks and ischemic strokes.

Corresponding author Frank A.J.L. Scheer, PhD, director of the Medical Chronobiology Program at Brigham and Women's Hospital (BWH), said that their findings suggest that the circadian system, or the internal body clock, contributes to the increased risk for cardiovascular events in the morning.

The researchers studied 12 healthy adult volunteers in the intensive physiological monitoring laboratories at BWH.

Participants were assessed throughout a two-week laboratory protocol designed to desynchronize daily behavioral and environmental rhythms from internal circadian rhythms.

Researchers specifically evaluated the role of Plasminogen activator inhibitor-1 (PAI-1), which inhibits the breakdown of blood clots and is thus a risk factor for blood clotting, one of the major contributors to heart attack and ischemic stroke.

The researchers sought out to test whether this morning peak in PAI-1 is caused by the internal circadian system or by behaviors that typically occur in the morning, such as altered posture and physical activity.

The researchers found a robust circadian rhythm in circulating PAI-1 with a peak corresponding to approximately 6:30 a.m. in a regular sleep/wake cycle.

Co-author Steven Shea, PhD, director of the Oregon Institute of Occupational Health Sciences, sad that their findings indicate that the human circadian system causes a morning peak in circulating levels of PAI-1, independent of any behavioral or environmental influences.

The new findings have been published in the journal Blood.