WASHINGTON, April 10 (Reuters) - Air traffic controllers should get more rest and step up safety vigilance, U.S. safety investigators said on Tuesday in recommendations prompted by a deadly Comair crash last year and other close calls.

"Controllers are sometimes operating in a state of fatigue because of their work schedules and poorly managed utilization of their rest periods between shifts, and that fatigue has contributed to controller errors," Mark Rosenker, chairman of the National Transportation Safety Board, wrote in a letter to the Federal Aviation Administration.

The safety board said nearly two-thirds of controllers work shifts with start times that progressively get earlier and change too fast to allow for proper rest.

Rapid rotation and short rest periods are likely reasons why controllers report sleeping an average of just 6.5 hours before day shifts and 2.3 hours before midnight shifts, Rosenker said.

Investigators pointed to the August Comair crash in Lexington, Kentucky, that killed 49 people and close calls in Chicago, Los Angeles, Denver and Seattle from 2001 to 2006 to illustrate their concern.

The Kentucky crash remains under investigation by the safety board. Much of the attention has fallen on the crew's use of the wrong runway -- an unlighted one that was too short to accommodate the 50-seat regional jet.

But control tower staffing and the lone controller's interaction with the doomed jet also have been scrutinized by authorities, who noted that he worked a day shift, left for nine hours, and then returned for overnight duty. The crash occurred toward the end of that all-night shift, at 6 a.m.

The controller said his only sleep between the two shifts was a two-hour nap. "Such limited sleep can degrade alertness, vigilance and judgment," Rosenker said.

In the investigation of a near-miss between a Boeing 737 and an Airbus A320 at Chicago O'Hare in March 2006, the safety board determined the controller at the center of the review had only slept about four hours between shifts.

"He reported that he felt 'semi-rested' but was 'not as sharp as he could have been.' The second shift had been a quick turnaround with 'no coffee,'" the safety board said.

The FAA had no immediate comment on the safety board's fatigue recommendation and another for the agency to expand training programs to enhance safety awareness in response to another series of close calls.

The union representing U.S. air traffic controllers, the National Air Traffic Controllers Association, could not be reached immediately for comment.

The union and the agency have clashed over appropriate staffing levels. Controllers complain that key facilities, especially in Southern California, are undermanned and their ranks are overworked. FAA officials say the air traffic system is adequately staffed.

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