Clea Howard is hardly a tuned-out, uninterested high school student. She likes to be busy: In addition to maintaining an excellent scholastic record at a demanding high school an hour from her Brooklyn home, she studies art, takes dance classes and plays soccer.

Yet during her freshman and sophomore years, she was always tired, no matter how much she slept at night. She often fell asleep in class, on the subway, while doing homework or talking to her boyfriend.

Even on vacation, when she logged 10 or 11 hours of sleep at night, Clea said, “I was still very tired during the day. I made excuses for myself — maybe I just need more sleep than other teenagers, or maybe I don’t feel any more tired than other people.”

Her pediatrician unearthed no medical reason or aberrant sleep habits to explain her extreme fatigue and tendency to doze off at the drop of a hat. An endocrinologist could not find any hormonal or diet-related abnormality. Perhaps, her pediatrician said, a sleep study might show if Clea was getting the kind of rest at night that restores body and mind.

At the Sleep Disorders Institute in New York, Clea met with a sleep specialist. It did not take long for Dr. Maha Ahmad to zero in on a possible diagnosis: narcolepsy.

“She asked me so many questions — 6 to 10 pages of probing questions — and showed me a brochure about narcolepsy that listed the symptoms,” Clea recalled. “I had them all.”

In addition to an uncontrollable tendency to nod off at inopportune times, Clea’s knees would buckle whenever she laughed or got very excited or stressed. If a minor disturbance occurred while she slept, like a paper dropping on her or her mother adjusting her covers, she woke up screaming and terrified, as if she were being attacked. And sometimes, as she was falling asleep or waking up, she would feel paralyzed, unable to move or speak.

Clea was relieved to know there could be an explanation for her inordinate sleepiness. But a definitive diagnosis depended on the results of a sleep study. Two were administered: a polysomnogram, an overnight test that monitors everything from heart and breathing rates to brain waves; and a multiple sleep latency test, a daytime exam of scheduled naps that checks for abnormalities in the sleep cycle.

In someone without a sleep disorder, it typically takes about 12 minutes to fall asleep; the rapid eye movement stage of sleep, so-called dream sleep, occurs after more than an hour of non-R.E.M. sleep. Clea’s test showed that she fell asleep almost immediately and quickly lapsed into R.E.M. sleep.

In normal R.E.M. sleep, muscles become paralyzed in a sense to prevent people from acting out their dreams. In someone with narcolepsy, the R.E.M. stage is often accompanied by muscle movements that result in restlessness and frequent awakenings.

The disordered nights are reflected in excessive daytime sleepiness, which in turn can cause mental fogginess, difficulty concentrating, lack of energy, depression, extreme exhaustion and sometimes memory lapses.

After their unavoidable naps, people with narcolepsy are only briefly refreshed. Within an hour or two, the uncontrollable sleepiness recurs.

Many Missed Diagnoses

Studies suggest that narcolepsy is far more common than most doctors realize. The Center for Narcolepsy at Stanford University estimates that the condition affects one person in 2,000. Most cases are undiagnosed and untreated. Misdiagnosis is very common, as well, with narcolepsy mistaken for laziness, depression, schizophrenia or an attention disorder.

“It usually takes about 10 years from the start of the disorder” for a case to be correctly diagnosed, Dr. Ahmad said. “Clea was very lucky — she was diagnosed rather quickly, in about two and a half years.”

Symptoms generally first appear, as they did for Clea, in adolescence or early adulthood, often following an environmental event, like a viral infection. Scientists have traced narcolepsy’s underpinnings to the autoimmune destruction of about 70,000 brain cells that normally produce a neurotransmitter called hypocretin.

With insufficient hypocretin, the brain is unable to properly regulate sleep, and the stages of sleep become disorganized. Instead of starting the night with 80 to 100 minutes of non-R.E.M. sleep, R.E.M. sleep begins within few minutes.

In one of the more bizarre manifestations of narcolepsy, during their mini-sleeps patients may continue to perform the activities they were doing while awake, albeit haphazardly, and they don’t remember doing them.

“I would fall asleep while I was writing and continue to write, but when I woke up, what I wrote while I was asleep was gibberish,” Clea said.

A Lifelong Disorder

Genetic variants, particularly in a gene involved in immune function, have been linked to narcolepsy. But while this variant occurs in a vast majority of people with narcolepsy, it also is carried by many others without the disorder.

The loss of hypocretin is irreversible, and narcolepsy is thus a lifelong ailment. Treatments have been developed that minimize the symptoms, but they require patients to stick to rather strict medication schedules and sleep behaviors.

Clea, for example, takes a long-acting amphetaminelike drug called Nuvigil during the day, which usually enables her to stay awake at school. She takes a second drug, a strong sedative called Xyrem, before bed and again four hours later to improve sleep quality and reduce daytime symptoms.

Most patients are told to also schedule one or more naps of 10 to 20 minutes during the day, something Clea finds challenging to fit in until she gets home after school or dance class. This may become easier, she said, when she starts college next fall, if she can create a schedule with down time between some classes.

Even though she has an incurable condition that she will have to deal with for the rest of her life, Clea considers herself lucky to be among those who receive a proper diagnosis and treatment.

She does wish “that more people would understand that I have a biological condition that requires medical treatment and stop giving me advice to try different diets, alternative remedies and what-have-you.”

Some even tell her she should just force herself to stay awake.

If only she could.