Love looks different on Matt Frerking. His eyes struggle to stay open. His neck gives in to gravity. His body slumps.
During an attack of love, Frerking can hear and feel everything going on around him. He is not in pain. But his eyes are closed. And he cannot move.
With cataplexy, a disease estimated to affect at least 50,000 Americans, many of whom suffer for years before they're diagnosed, the slightest trigger can cause an attack of paralysis.
For Frerking, 39, a stepgrandfather from Portland, Ore., the trigger for cataplexy is love. Love for his wife, Trish, love for his stepchildren and their children. He can also have an attack after watching strangers show love.
Love, that basic human emotion, becomes his prison.
If Frerking wants to avoid an attack, he needs to avoid the things most people long for, including physical affection and giving and receiving love. Even thinking about his feelings for others or watching strangers express love can trigger an episode of paralysis.
"I have to limit those things very carefully,'' Frerking told "Nightline's" Cynthia McFadden, in the first episode of a new "Nightline" series, "Secrets of Your Mind: Why We Do What We Do," which begins Thursday.
"The brain is behaving as if it's asleep while Matt is awake," said Carol Ash, a sleep specialist at the Sleep for Life Center at Somerset Medical Center in New Jersey. Ash has not treated Frerking but has seen many cases like his.
Typically, when a healthy person is sleeping, a switch is turned on to inhibit motor neurons and muscles.
"[It] inhibits the muscles at night so that you can't get up and start running in your sleep, turns on during wakefulness and suddenly you're paralyzed,'' said Ash. "And so in someone like Matt, strong emotions are flipping a switch."
The faulty "switch" in question is located in the brain's hyphothalamus region. No one knows why or how, but somehow, increased blood flow to the brain centers where emotions are processed -- the amygdala and the cingulate gyrus -- triggers attacks.
For Matt Frerking, thinking about his past can cause problems. Anniversaries are especially tricky.
Matt and Trish Frerking met working across the hall from each other in a laboratory at the University of California at Davis.
Just one photo from their wedding 13 years ago hangs on a wall in a bedroom of their Portland-area home. Most of their other photos are stashed away, out of view, in photo albums.
"It was a good day, and I think that's about all I can really say about it, so … let's head downstairs,'' said Frerking, not wanting to trigger an attack by dwelling on the memory, as he toured his home with ABCNews as his wedding anniversary was approaching.
As his wife flipped through their wedding album, she said, "That was a quick ceremony. You had a mullet."
Even these everyday moments are landmines for her husband. The combination of the photos and his wife's little joke brought on an attack.
"I love him,'' said Trish Frerking, 51, who keeps a blog about what it's like to live with someone who has cataplexy. "Frankly, he loves me or he wouldn't put himself through it. And I'm thankful I'm the person who gets to be with him most of the time. I still feel lucky 13 years later."