The incident happened in January at the Federal Aviation Administration control center in Palmdale, which handles high-altitude air traffic.
The worker was found napping at his work station and was suspended pending the outcome of disciplinary proceedings.
In all, the FAA has disclosed seven instances of controllers sleeping on the job in 2011 alone.
Meantime, Transportation Department Inspector General Calvin Scovel testified on Capitol Hill that the recent increase in errors is not just the result of better tracking systems.
Some people have theorized that a new program, encouraging controllers to report problems, may have simply caused a perceived spike in problems.
However, Scovel said even automated systems have recorded an increase. Overall, errors in which planes come too close together in the air increased 53 percent from 2009 to 2010.
The Associated Press contributed to this report.