(CNN) -- You may think the country's air traffic control towers are bustling with activity around the clock, but it turns out that it's a lonely job late at night for workers in some places when few planes are landing and taking off.
About 30 tower-controlled airports in the U.S. regularly have only one air traffic controller on duty on the overnight shift, said Laura Brown, a spokeswoman for the Federal Aviation Administration.
They include San Diego International Airport and Sacramento International in California; Tucson International Airport in Arizona; and Reno-Tahoe International in Nevada.
The staffing situation is now under extreme scrutiny after the troubling incident this week at Ronald Reagan Washington National Airport.
Two planes had to land without any communication from the control tower early Wednesday when the lone air traffic controller on duty on the overnight shift fell asleep, according to the National Transportation Safety Board.
Transportation Secretary Ray LaHood has now ordered the FAA to schedule two controllers on the overnight shift at Reagan and asked the agency to study staffing levels at other airports around the country.
On Friday, the FAA said it's instituting an interim plan to address all the concerns. The agency is reviewing overnight staffing at selected airports and reminding all air traffic controllers to offer pilots the option to divert to another airport if they do not make contact with the control tower
"I am determined to make sure we do not repeat Wednesday's unacceptable event," FAA Administrator Randy Babbitt said in a statement.
The National Air Traffic Controllers Association applauded the move.
"One-person shifts are unsafe. Period," Paul Rinaldi, the group's president, said in a statement. "Wednesday's incident clearly shows the need for an urgent, comprehensive, nationwide staffing study."
Having at least two people in the tower is a must, agreed Bob Richards, a retired air traffic controller who worked at Chicago's O'Hare International Airport for 22 years and is the author of the book "Secrets From The Tower."
If there is an emergency on board a plane, for example, one controller can deal directly with the aircraft and find out the details of the crisis, while the other controller can coordinate the emergency response on the ground, Richards said.
"Something can happen at any moment. Things can go wrong in a second and you have to be able to react to it," Richards told CNN's "American Morning."
He cited the crash of Comair Flight 191 in Lexington, Kentucky, that killed 49 people in 2006.
A federal investigation blamed the crash on pilots who mistakenly took off on a runway that was too short. But investigators also took a look at the actions of the lone air traffic controller on duty, who was working on just two hours of sleep at the time, according to the NTSB.
There's also the need for backup in case a controller himself has an emergency, Richards said.
"What if something happens to the person. What if he gets sick, has a heart attack?" he said.
System is 'incredibly safe'
But fliers should know the system is designed to function with no air traffic controllers on duty at certain times or in certain places.
There are thousands of airports of all types in the United States, and 90% of them don't have control towers, while many other airports have only part-time control towers, said Scott Wallace, the former director of accident investigations for the FAA.
Control towers at some fields across the country shut down for the night, and planes still land, said Peter Knudson, an NTSB spokesman.
Even airports the size of Reagan National see few flights late at night, so the number of workers in the tower is adjusted accordingly, Wallace said in an interview with "CNN Newsroom."
"Staffing levels are done typically based on traffic counts," he said.
"After midnight, departures are about zero, and arrivals are very few."
There are no scheduled departures at Reagan National after 11 p.m., while there is one scheduled arrival between midnight and 1 a.m. and another between 1 and 2 a.m., an aviation official familiar with the situation said. Traffic picks up again after 5 a.m.
The air traffic controllers on duty schedule breaks so that they're not out of the tower when they know a flight will be coming in, the official said.
The pilots who weren't able to get in touch with the tower at Reagan National on Wednesday always had the option of diverting to nearby airports, like Dulles International or Baltimore Washington International, or circling until establishing contact, the official said.
Meanwhile, Wallace believes the risk to passengers in the incident was very low.
"The system is just incredibly safe overall," he said.