Air travel is such an everyday experience these days that we are not surprised when we read about a politician having talks with the Japanese Prime Minister one day, attending a meeting in Australia the following morning and having to be off at midday to sign a trade agreement in Hong Kong. But frequent long-distance flying can be so tiring that the traveler begins to feel his brain is in one country, his digestion in another and his powers of concentration nowhere---in short, he hardly knows where he is. Air travel is so quick nowadays that we can leave London after breakfast and be in New York in eight hours, yet what really disturbs us most is that when we arrive it is lunch time while we have already had lunch on the plane and are expecting dinner. Doctors say that air travelers are in no condition to work after crossing a number of time zones. Airline pilots, however, often live by their own watches. The sentence "Airline pilots often live by their own watches." means  _  .
they don't trust others' watches they don't change their watches they make a living by their own watches they do as they used to do