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The engineer who drove a speeding commuter train off the rails in New York last year may have suffered from the most severe form of a dangerous sleep disorder, but health experts say he has plenty of company.

As many as 22 million people in the U.S. — or up to 7 percent of the population — may suffer from obstructive sleep apnea, experts say. It’s a condition that causes airways to collapse during sleep, cutting off breathing dozens or sometimes hundreds of times a night, leaving them bleary-eyed and drowsy, even after a full night’s rest.

William Rockefeller, 46, was diagnosed after the December 2013 crash that killed four and injured more than 70 with severe obstructive sleep apnea, documents released this week show. On a scale where as few as five sleep disruptions an hour can make someone sleepy, and 30 episodes are considered severe, Rockefeller logged about 66 arousals an hour, doctors said.

“His sleep was really fragmented,” said Dr. Phyllis Zee, a sleep expert with the Northwestern Medicine Sleep and Circadian Rhythms Research Program. “Even if he were to sleep seven or eight hours, he would be sleep-deprived.”

Zee and her colleagues suspected that Rockefeller might suffer from sleep deprivation. He was obese, records show, and there’s a certain fatigued look that she saw in news photos of the engineer.

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“That was one of my thoughts, ‘Oh my goodness, he has (OSA),’” she said.

In interviews after the crash, Rockefeller told investigators he was “dazed” and compared it to driving a car and staring at taillights in the distance, getting “that hypnotic feeling.”

Though he knew he snored, Rockefeller’s condition was apparently undiagnosed, according to National Transportation Safety Board records.

“His sleep was really fragmented. Even if he were to sleep seven or eight hours, he would be sleep-deprived.”

Those are classic signs of obstructive sleep apnea, Zee said. Though sufferers concede that they snore, most are unaware that they suffer from apnea, or interruptions in breathing, that cause them to rouse, but not fully. When this happens repeatedly night after night, they become chronically sleep-deprived and are at risk not only for chronic, life-threatening disease — but also the acute consequences of falling asleep during crucial tasks.

“For many of these people, not only do they have sleep apnea, they’re shift workers,” which also contributes to constant fatigue, said Zee.

A 2012 poll by the National Sleep Foundation found that more than a quarter of train operators admitted that sleepiness affected their job performance at least once a week.

The cure seems to be diagnosis and treatment, typically with a device called a CPAP — a continuous positive air pressure machine — which uses mild air pressure to keep the airways open. Response can be dramatic — and complete, Zee and other experts say.

In Rockefeller’s case, the engineer used a CPAP for 30 days, the documents said, adding that he was feeling “more energetic” and that his score on a popular sleepiness scale plunged.

The goal, Zee said, is to catch severe sleep disorders before such calamities occur.