For the last two and a half years, William Rockefeller worked nights. And on most of those nights, he was operating a Metro-North Railroad train along the Hudson line, the scheduled stops as familiar to him as the exit signs a driver sees day after day along a highway.
On Nov. 17, a new schedule went into effect. The next day, Mr. Rockeller began working a morning shift.
Less than two weeks later, at 7:19 a.m. on Dec. 1, the train he was operating derailed as it sped around a curve at 82 miles per hour just north of the Spuyten Duyvil station in the Bronx.
Immediately after the accident, Mr. Rockefeller had said that he was in a “daze” as the train gained speed, and on Monday, information released by the National Transportation Safety Board offered a possible medical explanation: undiagnosed sleep apnea.
As part of the safety board’s investigation, Mr. Rockefeller underwent a sleep study conducted by a board certified sleep medicine physician, who found that he had “high sleep fragmentation,” with breathing problems causing his sleep to be disrupted about 65 times an hour.
Such trouble sleeping was the result of “severe obstructive sleep apnea,” according to the report, a condition that had not been diagnosed by either his personal physician or by medical exams that were a mandatory part of his employment.
The sleep specialist noted Mr. Rockefeller’s change in schedule and found that “being a shift worker might have...