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Jet lag is an annoyance of modern life for which our pleistocene brain is completely unprepared. This ability to zip around the globe, to trapeze from time zone to time zone, is an invention of the late 20th century. Unfortunately, the brain is an organ of routine, equipped with a stubborn circadian clock. We are wired to expect a 24 hour day, and when our day extends far beyond that, the result is a set of symptoms that remind us we are far from home.
The problem of jet lag is also an interesting case study of stress. Hans Selye, the great Canadian endocrinologist, defined stress as the bodily response to any demand (stressor) that throws our body out of allostatic balance. (The response is an attempt to get that balance back.) Unfortunately for the globalized world, jet lag is one of those things that knocks us off balance. The end result is a large stress response, even if we don't typically associate duty free shopping, bad plane food and cabin boredom with stress.
This was made clear by a series of clever studies led by Kei Cho, a neuroscientist at the University of Bristol. He compared female flight attendants working for two different airlines. One company gave employees a 15 day break after working a transcontinental flight, while the other company gave employees a 5 day break. After controlling for a slew of variables, Cho found that the cabin crew working for the second airline - they were given fewer days to get over their jet lag - showed higher levels of stress hormone, impaired spatial memory and temporal lobe atrophy. This difference remained even when he controlled for the total amount of jet lag, suggesting that giving the mind time to adjust and recover is the crucial variable.
These upsetting results built on Cho's earlier work demonstrating that transmeridian travel leads to much higher levels of stress hormone (cortisol), both before, during and after the long flight. (The cabin crew didn't show higher levels of stress when working domestic flights.) This implies that the brain is bracing for its ordeal, preparing itself to deal with the arduous task of resetting its circadian clock. In fact, the mundane event appears so stressful that long haul flights cause menstrual disturbances in approximately 35 percent of female aircrew.
The moral, I suppose, is that modern life is full of rituals that clash with our internal machinery. (I'd wager that rush hour traffic is like getting jet lag everyday.) We obviously need transmeridian flights, but it's worth realizing that such journeys come with a real cost. And if we make these journeys rather frequently - if we're like those cabin crews working on little rest - then those costs just might be permanent.
