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One minute you’re feeling a bit woozy; the next thing you know, you’re flat on your back wondering what happened.

No matter what you call it — swooning, passing out or fainting — the experience is surprisingly common. About a third of people say they’ve fainted at least once.

Although often harmless, fainting can cause injuries, and sometimes signals a problem with the heart or circulatory system.

“Witnessing a faint can be scary because it can look like the person has died,” said Harvard professor Dr. Lewis A. Lipsitz, director of the Division of Gerontology at Beth Israel Deaconess Medical Center and the Institute for Aging Research at Hebrew SeniorLife in Boston. If it’s really a faint, the person will regain consciousness quickly; if not, call 911.

Here’s the lowdown on the most common reasons people pass out.

The “common faint”

These faints are caused by excessive stimulation of the vagus nerve, the part of the nervous system that regulates blood pressure and heart rate. In response to some trigger, the blood vessels in your legs relax and the heart rate slows, making it difficult for blood to return to the heart. Blood pressure drops, and down you go.

Sometimes, the trigger for vasovagal syncope, the official term for this kind of fainting, is a strong emotion — such as a response to bad news, the sight of blood, or a feeling of panic or claustrophobia. 

This type of faint can also occur when you have a bad coughing spell or strain to have a bowel movement, or even if you’ve been standing for a long time.

If you have any type of heart disease — including a previous heart attack, a rhythm disorder such as atrial fibrillation, a narrow or leaking heart valve, or heart failure — fainting may foretell a more serious problem and may require more extensive testing, Lipsitz said.

Julie Corliss is executive editor of Harvard Heart Letter.

© 2017. President and Fellows of Harvard College. All Rights Reserved. Distributed by Tribune Content Agency, LLC.