It would probably be best for Carol Bell if you didn't say anything clever when speaking to her. Your sharp wit could temporarily paralyze her.

The 67-year-old has a sleep disorder called cataplexy, which is a symptom of narcolepsy. (In other words, if you have narcolepsy, you don't necessarily have cataplexy. But if you have cataplexy, you definitely have narcolepsy.) For people with this condition, feeling a particular emotion causes them to briefly lose control of their muscles. For a few minutes, they're unable to move, although they're still conscious.

“People always ask me what it feels like,” says Bell, who lives in Edina, Minn., a suburb of Minneapolis. “I finally came up with an analogy: It’s like I’m a marionette. When the strings are cut, the marionette falls to the stage. And that’s exactly what it feels like. You no longer have control over your voluntary muscles.”

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During the REM stage of sleep, our muscles are essentially paralyzed. "That's a good thing -- we probably don't really want to act out our dreams," explains Dr. Michel Cramer Bornemann, a medical director at the Hennepin County Medical Center in Minneapolis, Minn. (Bornemann works at the sleep center where Bell is treated, although she is not a patient of his.) He explains that although the neurological mechanisms aren't totally understood, in patients with cataplexy, a particularly strong feeling appears to trigger that REM muscle paralysis when they're awake.

Bornemann asked me what I thought was the emotion most likely to trigger a "drop attack." I figured it would be something strong or even visceral, so I guessed anger. Nope. It's laughter -- specifically, laughter caused by telling your own joke.

For Bell, it's one-liners in conversations that do her in. When she first discovered she had cataplexy, she says, "I avoided social situations for a while. I was fearful of going places where I wasn't well known," Bell says. Now, her circle of friends is smaller than it once was, but they're all used to her occasional collapses. They'll sometimes continue their game of cards over her temporarily frozen figure. "The standard line is, 'Get a chair! She’s going down!'"

As she puts it, “You have to make a decision: Do you spend the rest of your life isolated, staying out of situations that might make you laugh and cause you to tip over?”

Bornemann has more stories of patients with cataplexy that are too fascinating to leave unshared:

A septuagenarian who can't make it to the punch line of a joke without collapsing.

A woman who is so overcome with emotion when holding her grandchild that her arms freeze. She’s come close to dropping the baby, but never has, and now that she and her family are aware of her condition, they're able to work around it.

The same woman, when she opens her front door to find a door-to-door salesperson, almost instantly tips over.

A man in his 60s who came face-to-face with a black bear in his backyard – the man fell over on his front porch, literally paralyzed by fear. (By the time he regained control of his body, the bear was gone. They do say to “play dead.”)

A man who freezes when he catches a fish.

A man who goes weak at the knees every time he sees a stray coin on the sidewalk.

Does anything make you go weak in the knees? Tell us in the comments.

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