Here's what's really going on during an episode of fainting, plus 5 reasons you might pass out.

During a live broadcast Monday morning, CNN anchor Poppy Harlow suddenly passed out on the air. While attempting to report on a new poll, Harlow started to slur her words before going totally silent. The show immediately cut to a commercial break, leaving viewers very concerned about the news anchor. After a barrage of worried tweets about her condition, Harlow reappeared on the air to voice that she was fine:

“For all of you on Twitter who are asking if I’m okay, thank you so much,” Harlow said. “I got a little hot, and I passed out for a moment. I am fine.”

The pregnant anchor also responded to the concerned messages on Twitter, ensuring everyone both she and the baby are safe.

While Harlow brushed off the incident coolly, there’s no denying fainting can be pretty frightening (especially when it happens on live TV!). Getting a little too hot isn't the only reason you might faint. Here's what's really going on, plus 5 reasons you might pass out.

Your blood pressure drops

Syncope is the technical term for fainting. "What’s happening, is for whatever reason, the oxygen or sugar supply to the brain has dropped off to the point that it shuts down," explains Melisa Lai-Becker, MD, site chief of emergency medicine at Cambridge Health Alliance. "Similar to when you fall asleep, your basic functions are preserved, your heart keeps beating, you keep breathing, but you lose consciousness."

Vasovagal syncope is one of the most common reasons for fainting, making up approximately 25-40% of cases. It occurs when the part of your nervous system that’s in charge of heart rate and blood pressure stops working due to some kind of trigger such as standing for long periods of time, overheating, seeing blood, having blood drawn, or even fear.

Any of these scenarios can cause a chain reaction that starts with a slowed heart rate. This leads the blood vessels in your legs to widen, allowing excess blood to collect in your legs, which lowers your blood pressure. As a result, there’s a sudden drop in blood flow to your brain, and that's what causes you to pass out.

This kind of fainting is normally not a sign of something serious.

You might have an irregular heartbeat

According to the Mayo Clinic, one fourth of all fainting episodes are related to abnormal heart rhythms, aka arrhythmias. This is especially true in cases of arrhythmia where the heart beats too fast (over 150 beats per minute), says Dr. Lai-Becker. When that happens, you may have plenty of blood in the body, but the heart does not allow enough of a relaxation period to send blood through the rest of the body and up to the brain.

Arrythmias aren't always serious, but they can signal an underlying problem that needs treatment. This is why it's important to see your doctor to get checked out if you have an episode of fainting.

You're hungry

At some point, you’ve probably experienced that awful light-headedness that comes when you’ve gone too long without eating, and this comes from a lack of fuel for your brain.

A drop in blood sugar can cause fainting because your brain has lost a major energy source, in this case, glucose. For diabetics and individuals with hypoglycemia (extremely low blood sugar), this can cause a reaction as extreme as a coma. "However, the average person who isn’t diabetic can still get shaky and sweaty [and even faint] if they haven't eaten enough," says Dr. Lai-Becker.

You’re overcome by emotion

We’ve all seen the stereotypical fainting scene in movies—you know, where a character is so shocked or emotional that he or she suddenly collapses. But can extreme emotion really cause fainting? Actually, yes.

"One common scenario is where someone may faint after hearing terrible or over-joyous news," explains Dr. Lai-Becker. "For example, some people faint at the point when we pronounce a loved one dead. It occurs when something comes over someone, emotionally and psychologically, and it’s too much to process." Fainting due to emotion, or psychogenic syncope, is even more common for individuals with anxiety, hysteria, panic, or major depressive disorders.

With that said, Dr. Lai-Becker points out: "Many physicians wouldn’t agree these scenarios are simply a result of overwhelming emotion, but rather a combination of factors." These other reasons may include anything from low blood pressure to hyperventilation brought on by extreme stress or anxiety.

Standing up

When you stand up, gravity causes blood to pool in your legs, which can decrease your blood pressure. "In this case, gravity is literally working against you, keeping the heart from pumpingblood fast enough up to your head,” says Dr. Lai-Becker. Normally, special cells in your body will sense the lower blood pressure and signal your brain to tell your heart to pump faster and your blood vessels to narrow; this process stabilizes your blood pressure so you don't feel woozy or faint.

But sometimes dehydration or even eating a big meal can disrupt this process (the technical term for this interruption: orthostatic hypotension), so that your body doesn't respond fast enough. This can cause you to faint if it's severe enough. During these situations, it's best to use gravity to your advantage rather than against you—put your head between your legs and allow the blood to flow back to your brain.

Feeling woozy or fainting after standing may also be a sign of a medical problem, so if it happens a lot or seems severe, see your doctor for a check-up, stat.