Sleepiness -- and with it loss of alertness -- are key factors in the growing number of major catastrophes that occur in the middle of the night, sleep researchers reported at the annual meeting of the Academy of Behavioral Medicine Research.

"We think that it's no accident that three of the major disasters that just happened -- Three Mile Island, Bhopal and Chernobyl -- all happened in the dead of the night on crews that were rotating schedules," said Dr. Charles Czeisler, director of the neuroendocrinology laboratory at the Harvard Medical School.

Sleepiness is "the inability of the brain to sustain wakefulness" in the face of little or no interesting activity or stimulation. It occurs while driving on a flat, monotonous stretch of highway or a familiar route home that requires little thinking. It also happens on the job, particularly during night shifts, and is an important factor in deadly mistakes that happen anywhere from hospital intensive care units to airplane cockpits and the control rooms of nuclear power plants.

One Harvard University study found that 30 percent of the operators of heavy equipment were falling asleep while driving on the night shift at a Utah company.

"I think that we have to understand what we are asking human beings to do," said University of Pennsylvania psychologist David Dinges said. "I think that we have to recognize that this alertness system has evolved over millions of years . . . Only in recent times have we structured this environment with artificial light and round-the-clock operations."

Among the recent major accidents occurring on the night shift when alertness drops to low levels are:

Two separate Burlington-Northern train accidents occurred in April 1984, each at 4 a.m.

The Dec. 26, 1985, accident at the Rancho-Seco nuclear plant, located about about 25 miles southeast of Sacramento, Calif. At 4:14 a.m., according to the Nuclear Regulatory Commission (NRC), there was a loss of electrical power to the plant's integrated control system. When the problem was over, 450 gallons of radioactive water had spilled and some radioactive steam had escaped.

At 1:35 a.m. on June 9, 1985, the Davis-Besse nuclear plant near Toledo shut down. The NRC said that auxiliary feed water pumps were activated, but then failed and resulted in a shutdown of the plant.

"We do see more accidents at night, and one of the things that concerns us is that we are seeing a disturbing number of problems in nuclear power plants at night," added University of Pennsylvania psychologist David Dinges. "I am not suggesting that these problems are necessarily caused by lowered alertness -- in some cases a mechanical failure originally precipitated the event -- but the question is, how long did it take the humans there to detect the problem and to correct the problem?"

"We are not suggesting that we stop shift work and night work," said Dinges. "We have to recognize the limitations . . . And we need to come up with countermeasures -- ways to help people stay awake."

Sleepiness peaks and alertness drops twice during every 24 hour period. Sleep scientists call these times windows of vulnerability -- periods when lack of sleep and the natural bodily rhythms conspire to make people drowsy despite their best efforts to stay awake. Nighttime sleepiness is strongest between 4 a.m. and 6 a.m. Daytime sleepiness peaks between noon and 6 p.m. In many cases, those who dose off go into "micro sleep patterns" -- they quickly fall into a form of sleep called stage one -- and are unaware "that they have been asleep," Czeisler said.

Added to the natural sleepiness pattern is the necessity of rotating shifts and working through the night -- both of which contribute to sleep deprivation. The problem is further compounded by the fact that few employers encourage workers to reveal trouble with staying awake on the job. "Airplane pilots are a profession where you don't report UFOs and you don't report lowered alertness," said Dinges. "All you're doing is asking for trouble if you do that."

How sleepy people get depends in part on how long they have been awake. "If you go with less sleep than normal for many days on end, you are likely to have alertness problems even during the day," Dinges said.

Sleepiness is also dictated by the body's circadian rhythms -- the natural cycles that, among other things, lower body temperature during the night and shift hormone levels. For instance, growth hormone levels rise during the night and peak just at the time when the body temperature is lowest and sleep is deepest. Blood levels of the hormone cortisol also drop.

"Alertness reaches its lowest point at the trough of body temperature," Czeisler said. These cycles operate whether or not a person sleeps.

"The reason that there aren't constant accidents on the night shift is that humans are pretty good at pushing themselves to get through it," Dinges said. "But add another stressor -- add no food, a foggy day, an alcoholic drink or a mechanical problem that is secondary to the issue of landing a plane or driving a train -- and now, in my view, the probability of catastrophe goes way up."

Toward morning -- and normal awakening -- the cycle advances. Growth hormone levels drop again. Body temperature rises, as does the blood level of cortisol. Many other functions in the body, including blood clotting, also change. Researchers have evidence that these circadian shifts could help account for the fact that heart attacks are three times more common from 6 a.m. to 9 a.m. compared to other times during the day.

For the night-time shift worker who has been up all night and must sleep during the day, circadian rhythms make life particularly difficult. "Even if you sleep during the day, your brain is saying, 'Hey, wake up,' so you don't sleep quite as long you might have slept at night," Dinges said. "So consistently we find that night workers have less sleep than day workers, and they are going to go through periods of lowered alertness at night on the night shift."

On weekends, night shift workers stay up during the day to be with their families. Yet just one exposure to sun on weekends resets the body's internal clock back to a day schedule. "So that Monday, when you go back on the night shift, you go back to living your life in jet lag," says Dinges.

Who adapts best to the night shift? In general, younger workers. There are also some people who only seem to need five hours of sleep a night.

"The public and our government agencies, our labor unions and our companies need to recognize this problem," Dinges said. "While we can never guarantee that there will not be a nuclear power plant accident or another airplane crash, we feel that we can lower the chances with public awareness. We're looking for countermeasures. We want to help people."