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 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 frontline FAA managers this month said a five-month internal review earlier 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 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 jet liners had to land on their own after the controller supervisor who was the lone man on duty fell asleep. A Knoxville controller working the overnight shift made a bed for himself and slept during a five-hour period when seven planes landed.

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

US Transportation Secretary Ray LaHood said he was outraged and 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.

LaHood last year instructed the FAA to work with the union on rules to ensure that the controllers who manage 24,000-27,000 commercial flights a day to arrive at work well rested.