Why Jet Lag Is Worse When You Fly East

Anyone who has flown overnight from Guatemala City to Madrid knows the feeling. You land in a city that is wide awake, the streets are full of people going about their day, and your body is convinced that it is the middle of the night. In winter, Madrid is seven hours ahead of Guatemala; in summer, when Spain puts its clocks forward and Guatemala does not, the difference grows to eight. Flying home again is unpleasant too, but most travelers find that the journey east is far harder to recover from. The reason lies in a tiny clock inside the brain.

Almost every organ in your body follows a daily rhythm, and a small group of nerve cells deep in the brain keeps them all in step, rather like the conductor of an orchestra. This master clock decides when you feel sleepy, when you feel hungry and even when your body temperature rises and falls. Curiously, it does not keep perfect time. Left to itself, without sunlight or clocks, it runs on a cycle that is slightly longer than 24 hours. Every morning, light reaching your eyes resets it, rather like someone nudging a slow watch back into line.

This small detail explains a great deal. Because its natural cycle is a little longer than a day, the clock finds it easier to be pushed later than to be pulled earlier. When you fly west, your day is stretched: you stay up a few hours longer than usual, which is roughly what your body would like to do anyway. When you fly east, the day is cut short. On your first night in Madrid, you are expected to fall asleep at what your body considers late afternoon. The next morning, you have to get up when, as far as it is concerned, it is still the middle of the night.

The US Centers for Disease Control and Prevention say that travelers adapt by about one and a half hours a day after flying west. After flying east, the figure is only about one hour a day. In other words, were you to cross seven time zones on the way to Madrid, your body might need around a week to catch up completely, while the return journey would take noticeably less. Until it does, you are likely to wake up at odd hours, feel hungry at the wrong times and find it hard to concentrate.

Fortunately, the same light that resets the clock every day can be used to speed things up. Bright morning light pushes the body clock earlier, which is exactly what is needed after an eastward flight, whereas bright light in the evening pushes it later. So, on your first day in Madrid, however tempting the hotel bed may look, resist the urge to sleep through the morning. Go outside, walk to a café and let the daylight do its work.