USA Today Friday said 3,404 of the nation's 15,475 air traffic controllers typically work the midnight shift at least once during a two-week pay period.
"Fatigue is a longstanding problem in the industry," National Transportation Safety Board Chairman Debbie Hersman said. "It shouldn't take controllers falling asleep on the job for the FAA to wake up to the fact that these schedules aren't in the best interest of safety."
Six controllers have been caught or been suspected of sleeping on the job at night or in the early morning since February. The most recent incident occurred Wednesday in Reno when a medical plane landed unassisted after failing to rouse anyone at the airport's tower.
The controller was suspended and the incident led to the resignation of Hank Krakowski, the FAA's air-traffic chief.
A report last month for the FAA and the National Air Traffic Controllers Association said, "Acute fatigue occurs on a daily basis due to reduced sleep opportunity."