Almost anyone who has flown across several time zones has experienced jet lag--the fatigue, mental fogginess and vague discomfort felt by travelers whose body clocks are out of sync with their environment.
But for some long-distance fliers, jet lag may be more than just a temporary annoyance. A new study of 20 flight attendants suggests that people who undergo repeated, frequent episodes of jet lag may develop impairment of visual memory and may even suffer some shrinkage of an important part of the brain.
If the findings are confirmed, they could have implications for airline employees and frequent travelers as well as shift workers, medical trainees and others who work long hours.
"The 8 million people who regularly work at night in this country may be experiencing some of the same phenomena," said Charles A. Czeisler, a professor of medicine at Harvard Medical School.
The research involved brain-imaging and memory tests on flight attendants who regularly traversed at least seven time zones.
So far, the data reveals that frequent jet lag without sufficient recovery time between trips affects the brain's structure and function, said Kwangwook Cho, a neurologist at the University of Bristol, England, who performed the study, which appears in the June issue of the journal Nature Neuroscience.
Cho is "finding actual tissue changes in the brain in an area that's involved in spatial orientation and related aspects of cognitive function," said Thomas Wehr, chief of the section on biological rhythms at the National Institute of Mental Health. "Probably more research would have to be done to tease out how much of that is related to sleep deprivation and how much is related to jet lag."
For the study, Cho recruited 20 healthy female flight attendants between the ages of 22 and 28 who worked for various airlines and had held their jobs for five years. All made regular flights across at least seven time zones, but half the women (the "short-recovery group") spent five days or fewer in their home time zone between long trips; half (the "long-recovery group") spent more than 14 days in the home time zone, working on shorter flights during that period. The number of hours spent on and off duty were comparable between the two groups.
For each participant, Cho took saliva samples to measure levels of cortisol, a hormone whose levels normally cycle in a daily rhythm but rise during times of stress. He also used magnetic resonance imaging (MRI), a type of brain scan, to measure the size of the temporal lobes, areas involved in language, memory and emotion. And he evaluated visual memory with a test that required participants to recall the location of black spots flashed on a computer screen.
In the flight attendants in the short-recovery group, the size of the right temporal lobe--a brain region key to visual and spatial memory--was slightly but significantly smaller than in the long-recovery group. The short-recovery group also performed more poorly and had slower reaction times on the visual memory test. Cho said that in an earlier study, he found that flight attendants' cortisol levels were high during long flights. He theorized that the stress of frequently changing time zones led to chronic elevation of cortisol in the short-recovery group, which may have contributed to changes in visual memory function.
Cho said he does not know whether the smaller size of the right temporal lobe in the short-recovery group reflected damage to nerve cells or a temporary, reversible alteration. But he believes that the brain needs at least 10 days to recover after a long trip.
"After they have more than four years in service, most cabin crew told me ... they notice their memory is getting worse," Cho said. "Clearly, we can prevent that kind of side effect simply by changing the [work] schedule" for flight attendants.