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Intricate detail emerges from these sleep studies, which show not only how deeply you sleep but also how much you snore, how your body moves during the process and how much you dream--and precisely when your dreams begin and end. Krakow, who reads the results with Talmudic intensity, can tell you exactly when you enter and leave rapid-eye-movement (REM) sleep, the state in which we become more aware of dreams. He can also tell you if you wake from REM sleep because of a leg jerk or a problem breathing even if you have no memory of waking up at all.
Over the years, Krakow began to notice that at least 90% of patients who went to him with persistent nightmares had either full-blown sleep apnea--a disorder in which your breathing pauses, sometimes for more than a minute, while you sleep--or a milder form of the disorder called upper-airway-resistance syndrome. He showed me several data sets collected after various sleep studies. In every case, the dream state was preceded by--or coincided with--an episode of disturbed breathing. "Breathing events can savage REM sleep," he told me. And when they do so, dreams can burst through the cover of REM sleep to become nightmares that wake us.
Krakow isn't sure why this happens, but one theory is that patients who have chronic nightmares have trouble getting oxygen to their brains during sleep. Because they are asleep, their brains cannot indicate to their bodies that the hippocampus is being starved. And so the body has evolved a way for an oxygen-deprived mind to awaken: nightmares. Krakow's conclusion: if you treat breathing disorders, you may eliminate nightmares.
Krakow is among a growing number of sleep doctors who believe that many if not most sleep disorders--not just persistent nightmares but also ordinary insomnia--are caused by the brain's reaction to apneas or other airflow limitations. And so over the past 20 years, the treatment of sleep problems has begun shifting from the psychological realm toward the pulmonary one.
The change has opened a vast new market in sleep medicine, which has become big business. The most common treatment for serious sleep apnea is the continuous-positive-airway-pressure machine, or CPAP (pronounced see-pap), which is sort of like a humidifier that pushes air into your mouth. The best CPAPs cost as much as $5,000. ResMed, the leading CPAP producer, is a publicly traded company that reported net income of more than $200 million in 2011. In the past few years, sleep doctors have also begun prescribing a drug called armodafinil to be used in concert with CPAPs. Armodafinil, a stimulant-like medication that patients can take in the morning to ensure they don't nap, can cost $500 per prescription.
Thousands of medical professionals in specialties such as cardiovascular care and ear-nose-throat treatment have begun to include sleep medicine in their practices--or have switched exclusively to sleep. In 1996, there were only 337 facilities accredited by the American Academy of Sleep Medicine; today there are more than 2,000. Earlier this year, the market-research giant IBISWorld estimated that sleep-disorder clinics in the U.S. earn a collective $6 billion in annual revenues--a figure that is growing 13% a year.