WASHINGTON — New regulations intended to keep air traffic controllers from dozing off on duty have been violated nearly 4,000 times, according to internal Federal Aviation Administration (FAA) documents.

After a controller fell asleep last year in the tower at Reagan National Airport, it emerged that such lapses were commonplace at airports across the country, and the FAA said it would act to curb the problem.

But a memo to more than 400 front-line FAA managers this month said a five-month internal review this year uncovered repeated violations of a requirement that controllers have at least nine hours off between shifts. More than half of the airport control towers were found to have violated the rule at least once. One facility broke the rule scores of times.

The review found the nine-hour-minimum rule was violated sometimes by minutes and other times by more than that.

The FAA suspended or fired several controllers for sleeping on the job last year, and the controversy contributed to the ouster of the head of the FAA's air traffic control organization.

Among those incidents was one at Reagan National Airport when the pilots of two late-night jetliners had to land on their own after the controller supervisor, the lone man on duty, fell asleep. A Knoxville, Tenn., controller working the overnight shift made a bed for himself and slept during five hours when seven planes landed. And a controller at a Nevada airport slept as a medical flight sought to land with a sick patient.

A scheduling practice that let controllers pack a full workweek into four days was singled out as the primary reason they were coming to work too tired to stay awake.

U.S. Transportation Secretary Ray LaHood put an immediate end to solo overnight shifts.

The FAA ordered that controllers have a minimum of nine hours off before a day shift and prohibited a popular shift-swapping practice that violated that rule.

"A vast majority of employees are meeting the requirement for nine consecutive hours of rest between shifts," said David Grizzle, FAA chief operating officer. "There are 12,000 shifts per month across the country, and in some cases, employees were arriving a few minutes early."

He said the FAA was updating its timekeeping software to prevent controllers from clocking in without nine hours' rest.

Gerald Dillingham, director of civil aviation for the U.S. Government Accountability Office, described the nine-hour break as an important standard, but said he wasn't surprised to learn it was being violated.

One veteran controller who asked not to be named described the nine-hour requirement as "more of a nuisance than a true answer to fatigue mitigation."