Is Jet Lag All in the Mind?

LeAnn Sawabini never wears a watch to work. Looking at the time doesn’t help her do her job any better, she said. And yet time affects Sawabini’s life profoundly.

As a flight attendant who often sleeps in three different cities as often as twice a week, she’s used to having her rhythms regularly thrown off track and her sleep patterns badly disturbed.

But if a recent study is correct, chronic jet lag might cause more debilitating effects than simply tiredness.

The study, conducted by Kwangwook Cho, a scientist at the University of Bristol in Britain, compared two groups of female flight attendants that both experienced chronic jet lag. One group had more than 14 days to recover between long-haul flights through multiple time zones. The other had a recovery period of less than five days.

Cho found that the second group performed more poorly on a test of mental ability. And their saliva contained greater amounts of a hormone, cortisol, that is associated with stress.

Most strikingly, Cho found that a part of the brain called the temporal lobe, a structure involved in the formation of memories, was smaller in the group of flight attendants with the shorter recovery time.

The study isn’t meant to alarm, said Cho. Rather, he said, it suggests that flight attendants and other workers experiencing regular jet lag need longer recovery times between trips through many time zones.

The study “raises some important public health issues,” said Christopher Colwell, assistant professor in the department of psychiatry and biobehavioral sciences at UCLA.

Scientists (and travelers) have long known that when people fly rapidly through time zones their sleep patterns go haywire for a while because their biological clocks are disrupted.

This biological clock regulates many functions of the body. People sleep, wake and eat in rhythms.

Each day, the clock is reset by light to keep it in sync with the outside world. But when we move rapidly through time zones, light can’t reset the clock fast enough and we experience jet lag until our clocks adjust.

For many people, experiencing jet lag is limited to the occasional business trip or a dream vacation in Europe. That isn’t so for flight attendants such as Sawabini or for airline pilots.

Other groups of people may also be harmed by frequent disruptions of their sleep rhythms.

The research “winds up being instructive for people doing long periods of phase-shifted work like medical residents,” said Robert Sapolsky, associate professor of neuroendocrinology at Stanford University.

In fact, shift workers whose jobs require them to wake and sleep at unnatural times have similar problems to those enduring chronic jet lag, said Charmane Eastman, director of the biological rhythms research laboratory at Rush Presbyterian-St. Luke’s Medical Center in Chicago. Some of the biggest industrial disasters (such as the partial meltdown of the nuclear power plant at Three Mile Island) have happened during shift workers’ watches at times when people are normally asleep, she said.

That means there are important safety implications in figuring out how changes in the body’s internal clock affect the brain, said John Gabrieli, an associate professor in the department of psychology and neurosciences at Stanford University.

To determine the effect of jet lag on flight attendants, Cho gave them a memory test in which they were shown four target locations on a screen and later had to recall those locations. The group with a longer recovery period between flights responded about 80 milliseconds quicker than the second group--a significant difference, Cho said.

The study, published in the June issue of Nature Neuroscience, also looked at levels of cortisol, a hormone released by the adrenal glands during stressful life events. The hormone is involved in the “flight-or-fight” response, said Sapolsky. It prepares the body for short bursts of activity, for instance, by mobilizing energy to muscles and increasing blood pressure.

Exposure to cortisol over long periods of time can have an adverse effect on the body, Sapolsky said. It increases people’s risk for high blood pressure and heart attacks.

Cho also measured the volumes of the flight attendants’ right and left temporal lobes using a brain-imaging technique called MRI. He found that the flight attendants with the short recovery period had almost 2% less tissue in their right temporal lobes than did flight attendants in the other group. (There was no differences in the left temporal lobes.)

This fits with other studies showing that high cortisol levels can reduce the size of a part of the brain called the hippocampus, said Sapolsky. The hippocampus, which is important for memory formation, is in the temporal lobe.

It is also known that damage to the temporal lobe can result in the inability to form new memories, said Gabrieli. Damage to the right temporal lobe in particular results in the inability to form new spatial memories.

That finding might explain why the group of flight attendants with a reduced temporal lobe didn’t perform as well on the spatial test, said Gabrieli.

Cho is not saying that people with frequent jet lag will sustain brain damage. But his study does imply negative effects from confusing the body’s biological clock over a prolonged period of time, he said.

Chronic jet lag, he proposed, leads to sleep loss, stress and overproduction of cortisol. Cortisol, in turn, might cause changes in the brain that affect thinking ability.

Airlines should give flight attendants at least 10 days of recovery between flights through multiple time zones, he suggested. That would give people more time to adapt to the zone they are in so that they are more rested and less stressed.

Cho plans to replicate the study with male flight attendants and pilots as well as nurses, who don’t have chronic jet lag but have highly stressful lives and often work night shifts.

Other researches offered cautionary notes about the findings. The study needs to be replicated, said Dr. Al Lewy, director of the sleep and mood disorders laboratory at Oregon Health & Science University in Portland. Also, he said, it’s important to make sure factors other than jet lag, such as sleeping pills or alcohol, aren’t responsible for the brain changes.

Dr. Clifford Saper, professor of neurology and neuroscience at Harvard Medical School, pointed out that the group of flight attendants who had a 14-day recovery period before flying through multiple time zones also flew shorter flights in the interim. The group with the five-day recovery time flew fewer of these short flights.

Thus, Saper said, the differences Cho found might be caused by a difference in stress that had nothing to do with time zone shifts but merely how many short or long flights people flew.

Saper also termed the 2% difference in right temporal lobe volume “trivial and almost certainly meaningless.”

For her part, Sawabini says she’s sure that after four years as a flight attendant she doesn’t have any memory loss. But what about those who’ve been on the job longer?

Recently, she said, she was talking with some older flight attendants. One of them was going through a common experience--she had misplaced her glasses and was having a hard time finding them, she said. The woman had been a flight attendant for 30 years.

“Watching her, I wondered if her memory was affected because of age,” said Sawabini. “Or was it because she had been flying too long?”