There have now been seven controllers believed to have fallen asleep on the job on the midnight shift since the start of the year.
The Federal Aviation Administration said another controller fell asleep early Saturday morning at a radar facility in Miami. Officials suspended the controller.
Controllers often bounce from morning shifts to afternoon and night shifts, leaving little time for the body to adjust.
A recent study found that for controllers "Acute fatigue occurs on a daily basis" and "Fatigue can occur at any time, on any shift." That's something the head of the union acknowledged this week.
"We have a fatigue issue," said Paul Rinaldi, president of the National Air Traffic Controllers Association. "We have a high stress occupation that works around the clock, 24 hours a day, seven days a week."