Julie Flygare was studying art history at Brown University when the weird symptoms started: Her knees would buckle when she laughed or got angry.
"I felt like I was melting inside," said the now 28-year-old who lives in Washington, D.C. "And it got worse and worse with more emotion."
Keeping feelings at bay became impossible with her boyfriend. During romantic encounters, her whole body began to collapse.
"My head started giving out with the sexual excitement," said Flygare. "My head feel backward on the pillow like whiplash."
Flygare had vivid hallucinations and sometimes she lay in bed aware of her surroundings, but paralyzed.
"No one seemed to know what it was. I was so confused," she said.
Was she having a seizure or a stroke -- or even mental problems?
"The symptoms can sound like a crazy person," said Dr. Mark Dyken, director of the Sleep Disorders Center, University of Iowa Hospitals and Clinics. "It could be someone with major psychotic depression with catatonia, but in fact it's narcolepsy."
Narcolepsy is a neurological sleep disorder in which the brain loses its ability to maintain normal sleep and wake states.
Scientists like Dyken speculate that it may be genetic in nature, triggered by an autoimmune response.
Narcolepsy affects more than 200,000 Americans -- as many as those with multiple sclerosis, according to Wake Up Narcolepsy, an organization that supports research.
Flygare has the most serious form, narcolepsy with cataplexy -- a sudden loss of muscle tone while awake, resulting in the inability to move.
These episodes of temporary paralysis are triggered by emotion, and she is fully conscious throughout.
"I hear everything going on around me -- people saying, 'Oh my god, oh my god'," she said. "I am lying there and I can breathe and my heart is going."
Given its social implications, narcolepsy is as disruptive to daily life as epilepsy and Parkinson's disease, but is often misdiagnosed by doctors and misunderstood by friends and family.
"There is a lot of stigma," said Shelby Harris, director of the Wake-Sleep Disorder Center at Montefiore Medical Center in New York City. "People go 10 or 15 years before being diagnosed and they think they are lazy or depressed. It causes rifts in families and on the job."
Most often, it is diagnosed in a person's 20s or 30's, but Harris said she just saw a patient who was 15.
Its hallmark symptom is hypersomnia or being sleepy all the time, according to Harris. But during the night sleep is often disrupted.
"They can also have things called sleep attacks, where they fall asleep in a warm setting," she said.
In cataplexy, the muscles weaken. "A lot of people report that their legs buckle or their face droops or half their body is weak," said Harris. "It's brought on by strong emotion -- positive or negative."
"It can be scary for others looking at you," she said. "They think you've passed out or had a stroke. Often it's about educating people around you. But you are fine, it will pass."
Freelance writer Meghan Holohan describes the perils of dating with narcolepsy. She can feels the cataplexy coming on as she enters her boyfriend's apartment anticipating sex.
"I have 30 seconds to make it to my couch or I will lose consciousness and crash onto the floor," she writes in a recent column for Salon magazine, "Dating With Narcolepsy."
"Nathan stands and I charge through the front door, lumbering through the hallway like Godzilla," she writes. "I nearly trample my beagle, who squeals like an unsuspecting villager does when the killer lizard comes to town. I consider crawling to my couch, but I am also trying to impress Nathan.
"If I fall unconscious, I could be out for up to 10 minutes. From my experience, men don't take it well when you collapse without explanation."
Narcolepsy Is Socially Isolating
Holohan first collapsed at 12 on top of a boy she had a crush on.
"I once rushed out of a bar after flirting with a study partner and fainted on the steps of a church," she writes. "I once passed out in an alley, banging my head on a garage door, waking up in a puddle beside a guy I was dating, who said, 'I thought you were dead!'"
Holohan, 33, has also been accused of being drunk at a party. "I find anything is easier to deal with using humor," she told ABCNews.com.
Flygare knows how socially isolating narcolopsy can be and doesn't attend large parties anymore.
"The challenges are huge and they are internal," said Flygare. "People hear the word narcolepsy and get weird reactions -- people don't think it's a serious thing."
Experts work with narcolepsy patients and their families to avoid situations that could be dangerous.
"New moms should breast feed lying down, so they don't fall on top of the child," said Harris. "If you are walking with a child around the house, push them in cart."
Narcolepsy is a lifelong condition, but treatable.
Flygare takes four medications a day -- including one in the middle of the night -- and must take twice-daily naps. She wears a special bracelet that alerts people about her condition and who to call.
"I was angry for awhile, sad and depressed -- it was really hard," she said. But it hasn't stopped her from living a full life.
In 2010, she completed the Boston Marathon in four hours and 41 minutes, raising $6,000 for charity. Today, she writes a blog, REM Runner. Flygare has also shopping around her memoir, "Wide Awake and Dreaming," to publishers.
And in June, she will compete in the Mount Washington Road Race in New Hampshire, a seven-mile, 4,500-foot climb.
She tries to keep her emotions in check so as not to trigger an episode. While running the marathon, she kept her music going and looked to the ground.
"People think a marathon is difficult, but it doesn't compare to the daily struggle of living with narcolepsy," said Flygare.
But she challenges herself to be optimistic about her future and has found working part-time for Wake Up Narcolepsy rewarding.
"I really enjoy doing awareness [work] and I am a little bit less afraid," said Flygare.
As for the boyfriend who watched as she went limp on date, Flygare said he broke up with her. But she knows one day, a special guy will appreciate her unique disorder.
"For a long time, I kept it a secret," she said. "But now I am a spokesperson for narcolepsy. A guy can Google my name and I can't really hide it."
"I have overcome adversity, and the right kind of person will find me fascinating," she said. "To me, it's a good way to weed the guys out early."