A study – kept secret for almost four years by the Federal Aviation Administration (FAA) – shows that air traffic controllers are a potential danger to travelers because their work schedules lead to chronic fatigue, reports Fox News.

The study, which was dug up by the medai after the FAA refused to honor a Freedom of Information Act request to provide a copy of the report, found that 2 out of 10 controllers committed errors while on the job within the span of one year. Over half of these errors were caused by fatigue. More than 6 in 10 controllers said they had fallen asleep or were distracted on their way to their shifts.The controllers averaged approximately 5.8 hours of sleep per day. Air controllers are habitually forced to work six-day workweeks, including midnight shifts.

Approximmately 3,268 controllers were surveyed to compile the results, according to The Miami Herald. A separate field study monitored alertness and sleep habits in more than 200 controllers across 30 air traffic facilities.

The study concluded with 17 recommendations for the FAA, including one to cease six-day work weeks.

The study was produced by NASA, according to Time, following a recommendation from the National Transportation Safety Board.