Jet Travel Over Many Time Zones Upsets Pilots’ Built-in Clocks

When pilots struggle to hold their eyes open on late-night flights, they are fighting the rhythms of their own bodies, the built-in timekeepers that rule their cycles of sleep and alertness.

Pilots’ schedules often require them to get up early, stay up late or fly at times when their bodies tell them they should be fast asleep.

Sleep experts say the effects on the pilots are clear: They grow agonizingly drowsy when they should be alert, but at bedtime they frequently have trouble nodding off. The result is a potentially dangerous combination of inattention and cumulative sleep loss.

“Just as we want our planes maintained so mechanical factors don’t crash them, we should equally be concerned about the maintenance of our pilots so biological problems don’t crash them,” said Dr. David Dinges, a sleep expert at the University of Pennsylvania.

Body Can’t Keep Up

When people traveled by ship, they crossed a maximum of one time zone a day. Their bodies adapted easily. Now jets span five or six time zones in a single night. The human body cannot keep up.

Under a fairly typical long-haul schedule, a crew might fly from San Francisco to London one day, to Seattle the next, then back to London to Seattle to London to Seattle. In eight days, they cross time zones 48 times.

A vacationer landing in London after one of those legs might feel woolly for a couple of days, but no harm is done. However, for the pilot crisscrossing the planet, there is no chance to recover from jet leg. The price: A spiraling sleep debt.

Although flights that span continents and oceans provide the most dramatic examples of sleep disruption, shorter domestic trips can be exhausting too. A pilot’s work day can last 16 hours. Experts say that after a late-evening arrival, there often is simply not enough time for a decent night’s sleep between check-in at the hotel and the trip back to the airport for the next day’s duty.

If a pilot gets just one hour less sleep than normal each night, the debt after several days can significantly weaken attention, diligence and motivation.

“Pilots have disrupted sleep and significantly shortened sleep while away from home,” said Dr. Martin Moore-Ede, a sleep expert at Harvard Medical School. “Over longer distances . . . they find themselves fatigued, are inalert in the cockpit and drop off to sleep.”

Pilots, of course, are not alone in this. Single-vehicle traffic accidents, in which cars and trucks simply veer off the road, are most likely to happen in the middle of the night.

So are industrial accidents. The nuclear power plant disasters at Three Mile Island and Chernobyl resulted from mistakes made in the wee hours.

In the last two decades, many experiments on volunteers have documented the effects of working and thinking while short of sleep or out of synchronism with the body’s built-in clock, the circadian system.

This research underscores the belief that fatigue is as risky for pilots as it is for nuclear plant operators and long-haul truckers or anyone else who must make life-and-death decisions. But proving this in the cockpit is difficult.

No blood test can measure sleepiness after a crash or near miss. Even if one could, a crash almost always results from a series of mistakes and bad luck; fatigue may be only one link in the chain of disaster. So a few sleep experts have begun flying with pilots to see what really goes on.

“I’ve been all over the world with these guys. I know what happens,” said Dr. R. Curtis Graeber, a psychologist at the NASA-Ames Research Center at Moffett Field in California.

“We’ve seen fuel mismanagement, two engines being flamed out and nobody could figure out why for 20 minutes,” he said. “We’ve seen tires blown because the brakes were mis-set.”

Was fatigue to blame? “There’s no smoking gun,” Graeber said. “I can’t prove to you that the guy mismanaged the fuel because he was tired. But I can tell you that he did it on the segment in which we had seen the greatest fatigue. We don’t tend to see those errors on the first flight of the trip.”

Graeber and others hook pilots and engineers to monitors while they fly. They hope to see whether crew members make more mistakes when such physical signs as body temperature and heartbeat suggest they are less alert.

Much of that data is still being collected and analyzed. But Graeber said it’s clear that by the third night of such schedules as the San Francisco-London-Seattle tour, the cockpit crew becomes increasingly lethargic.

Dr. Mary Carskadon of Brown University has shown in the laboratory what Graeber sees in the cockpit: The effects of lost sleep accumulate over time.

“Most people, particularly those who are trained as pilots are, can cope with the effects of chronic sleep deprivation fairly well most of the time,” she said. “But . . . a vulnerability is set up that puts them at risk for catastrophic events to occur.”

Experts say that when people are tired, they become sloppy. Motivation flags, inertia sets in. They take shortcuts and ignore small details.

Flying a big plane, however, is mostly a matter of paying attention to small details--setting instruments, following checklists, getting weather reports, watching dials.

“It’s the errors of omission that occur because of lapses in attention that are the hallmark of fatigue-related accidents,” said Dr. Charles Czeisler of Harvard Medical School. Such omissions would include not “noticing an indicator that’s not right, (not) anticipating a problem before it occurs, (not) thinking many steps down the line,” he said.

Sleep Is Contagious

Sometimes pilots and flight engineers fall asleep. On long cruises, especially over oceans, this is not terribly dangerous as long as someone in the cockpit stays awake. But sleep is contagious. The sleep experts say their research has turned up many tales of entire crews nodding off.

Another concern is micro-sleep, a shadowy state in which people are half asleep. With the brain waves of sleep intruding on those of wakefulness, people in this condition can continue to perform duties while only vaguely aware of what’s going on around them.

“It’s much worse than being, frankly, fully asleep, because they can continue to carry out routine tasks,” said Czeisler. “If they were truly asleep, someone else at least might notice.”

These problems occur, in part, because jet travel ignores the circadian timekeeper, the product of millions of years of evolution on a planet rotating once each 24 hours. Each day, the clock oversees a symphony of changes in body temperature, alertness and hormonal secretions, all of them geared to roughly a 24-hour cycle.

For most people, the daily swing reaches its low point from 3:30 to 5 a.m. This is when they are sleepiest. There is another, less pronounced, valley of drowsiness in the afternoon from 4 to 5. The body temperature is also lowest at these times.

Consider the pilot who leaves California on a late-night, cross-country flight. Even if he slept during the day, he’s likely to feel sleepy early in the morning as his body temperature falls. By the time he checks into a New York hotel, his body temperature is rising again--his body clock is telling him to wake up. Even though he is tired, sleep is difficult. He awakens feeling ragged and has to fly again.

“The pilot who flies across multiple time zones faces the problem that his body continues at the old time zone,” said Dinges. “He is forced to try to sleep when his body clock wants him to be awake and to be awake when his body clock wants him to sleep.”