Many travelers notice that jet lag feels worse after flying east than after flying west. The difference comes from the way the body keeps time.
Deep inside the brain, a small group of cells works as a master clock. It controls when we feel sleepy, when body temperature rises and falls, and when certain hormones are released. However, this clock does not run on exactly twenty-four hours. In a careful study published in 1999, researchers at Harvard Medical School kept volunteers away from normal daylight and measured their internal rhythms. They found that the human clock runs, on average, for about twenty-four hours and eleven minutes.
Because the natural cycle is slightly longer than a day, the clock must be reset a little each morning. Light does most of this work. Morning light entering the eyes tells the clock to move slightly earlier, so that it stays in step with the sun.
A long flight suddenly moves the traveler into a different time zone. Flying west makes the day longer, so the body must go to sleep later than usual. The clock can do this fairly easily, since it already tends to run late. Flying east makes the day shorter, and the body must fall asleep earlier. This asks the clock to move in the direction it does not naturally go, which is why adjusting often takes longer.
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