by Richard N. Fogoros, M.D.

QUESTION: I've heard something about most heart attacks occurring early, probably in relation to early-morning workouts and water intake. If a connection does exist, could you please explain what it is, or how it works?
—Maxwell, New York

ANSWER: You've hit on a fascinating aspect of cardiovascular medicine—and one that is fairly well documented.

The risk of having a heart attack is about 40-percent higher between 6 a.m. and noon. Other cardiac events, such as cardiac arrest and stroke, are also more likely in the morning.

We're not positive why this is, but there are some strong theories about what contributes to it:

• A rise in adrenaline levels. This probably happens because of many factors, including just getting geared up for daily activities. Also, you're relatively dehydrated in the morning because you've been sleeping, so your body starts drawing water from the bloodstream. This makes your cardiovascular system work harder to pump blood, and adrenaline is one substance that helps stimulate it to do so.
• An increase in blood-clotting factors, at least in some people. Blood clots are generally what cause heart attacks.
• Higher blood pressure. Blood pressure drops at night and returns to usual in the morning. If a heart attack or stroke is about to occur, it makes sense that it would more likely happen in the morning, when the cardiovascular system is getting going, than at night, when it is at relative rest.

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.

RICHARD N. FOGOROS, M.D., is About.com's guide to heart disease and cardiology. He's also consulting medical advisor for research and development and consulting patient safety advisor for Boston Scientific CRM, which designs implantable medical devices.

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