Chicago ? The nuclear power industry is so shorthanded that workers often put in numbingly long hours on the job, with critics warning that safety at nuclear plants could be endangered by employee fatigue.

Union officials and plant workers say that overtime, sometimes a remarkable numbers of hours, has increased in recent years as the pool of skilled employees shrinks. The workload has grown so onerous that the Nuclear Regulatory Commission, giving consideration to safety issues, is weighing whether to tighten regulations on how long people can work.

In an official document issued recently, the staff of the NRC states that industry work scheduling policies do not ensure “that personnel are not impaired by work-related fatigue.”

Concerns about overwork date to the late 1990s, when the Union of Concerned Scientists and others asked the commission to limit work hours to prevent excessive fatigue that could compromise safety.

“The NRC must establish clear requirements for working hours that reduce the potential for weary workers making grave mistakes,” the Union of Concerned Scientists said in 1999. The group says it has the same concerns today as it did then.

The overtime stems from a declining workforce.

The Nuclear Energy Institute says nuclear plant workers, excluding security personnel and contractors, numbered 56,400 in 2002. In 2003 that dropped to 55,700. In 2004 it dropped again, to 53,750.

The International Brotherhood of Electrical Workers, which represents nuclear workers in Illinois and other parts of the country, said staffing has been declining for years.

An official at IBEW Local 15 said, for example, that at the plants it represents in Illinois, its membership has fallen from 2,175 members in 2000 to 1,525 this year.

The NRC has been studying the issue of worker fatigue since 1999, with no certain indication of when a decision will be made. The long delay is attributed in part to the agency having to deal with security concerns at nuclear plants after the Sept. 11 attacks.

A nuclear plant is a remarkably complex machine that runs full speed, 24 hours a day, seven days a week. It demands constant human attention and meticulous maintenance.

The plant sets the schedule for the people who operate it.

The lights in a reactor control room are never turned off because people are always at work there. The safety response team must be on the job at 3 a.m., just as it is at 3 p.m. The worker whose job is to make sure no one is overexposed to radiation can’t leave work until his replacement arrives, no matter the time of day or night.

Todd Newkirk studies fatigue and work-hour issues for the IBEW, and he says overtime is increasing.

“It’s a fallout of electrical deregulation,” he said. “It is doing more with less.”