Mornings between 6 a.m. and noon are high risk hours for heart attack, cardiac arrest and stroke. The risk for heart attack is 40 percent higher during those hours.
It is speculated that this high risk level may be linked with higher adrenaline levels in the mornings. Upon awakening the body is pulling water from the blood stream so the cardiovascular system has to work harder in the morning. There is also a possible increase in blood clotting factors. And blood pressure is higher in the morning and lower at night.
"In any case, no matter what time it is, heart attacks don't happen at all in people who don't already have coronary-artery disease and aren't an ant's eyelash away from sudden coronary-artery blockage. Therefore, the key to preventing this phenomenon is not (for instance) to drink extra fluids at night; it is to control aggressively any risk factors you may have for developing coronary-artery disease in the first place."