第4部分：阅读理解（第31～45题，每题3分，共45分）
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The Body Clock
Why is it that flying to New York from London will leave you feeling less tired than flying to London from New York? The answer may be a clear case of biology not being able to keep up with technology.
Deep inside the brain there is a 'clock' that governs every aspect of the body's functioning: sleep and wake cycles, levels of alertness, performance, mood, hormone levels, digestion, body temperature and so on. It regulates all of these functions on a 24-hour basis and is called the biological clock.
The body clock programmes us to be sleepy twice a day, between 3-5 am and again between 3-5 pm. Afternoon tea and nap are all cultural responses to our natural biological sleepiness in the afternoon.
One of the major causes of the travellers' malady known as jet lag is the non-alignment of a person's internal body clock with clocks in the external world. Crossing different time zones confuses the biological clock, which then has to adjust to the new time and patterns of light and activity. To make matters more complex, not all internal body functions adjust at the same rate. So your sleep/wake may adjust to a new time zone at one rate, while your temperature adjusts at a different pace. Your digestion may be on a different schedule altogether.
Though we live in a 24-hour day, the natural tendency of the body clock is to extend our day beyond 24 hours. It is contrary to our biological programming to 'shrink' our day.
That is why traveling in a westward direction is more body-clock friendly than flying east. NASA studies of long haul pilots showed that westward travel was associated with significantly better sleep quantity and quality than eastward flights.
When flying west, you are 'extending your day, thus traveling in the natural direction of your internal clock. Flying eastward will involve 'shrinking' or reducing your day and is in direct opposition to your internal clock's natural tendency.
One of the more common complaints of travelers is that their sleep becomes disrupted. There are many reasons for this: changing time zones and schedules, changing light and activity levels, trying to sleep when your body clock is programmed to be awake, disruption of the internal biological clock and working longer hours.
It is often suggested that you adjust your watch as soon as you board a plane, supposedly to try to help you adjust to your destination's schedule as soon as you arrive. But it can take the body clock several days to several weeks to fully adjust to a new time zone.
