Heart attacks, strokes, and cardiac arrests seem like they come out of the blue, but most don’t. They usually appear after cholesterol-rich plaque has festered in the arteries that nourish the heart and brain. So what makes one happen at a particular time? A trigger, reports the July 2007 issue of the Harvard Heart Letter.

Important triggers include:

Waking from sleep. Before you wake up, your body trickles stress hormones into the bloodstream. This helps you get up, but also slightly stresses the heart. That, along with dehydration that occurs overnight and the overnight fade in protection from heart medicines, may explain why heart attacks are most common in the morning.

Heavy physical exertion. Shoveling snow, running, and other strenuous activities can be triggers. But don’t be afraid to exercise—exertion is much less likely to cause trouble in people who exercise regularly.

Anger. A bout of anger can increase the chances of having a heart attack up to 14-fold during the following two hours.

Infections. Pneumonia, flu, and other infections can be potent triggers for heart attacks and strokes.

Other triggers include sexual activity, overeating, severe hot or cold weather, air pollution, natural disasters, drug use, grief, and lack of sleep.

Of course, most people with heart disease get out of bed in the morning, shovel snow, make love, get angry, and suffer through the flu just fine. Still, knowing what sets off heart attacks, strokes, or cardiac arrests can help you avoid triggers or blunt their power, says the Harvard Heart Letter.