1 Humans have biological clocks that control their daily rhythms. Some
2 functions, like sleeping and waking yourself up have a 24-hour rhythm.
3 Others, especially the illnesses, have a yearly rhythm. Cold and flus, for
4 example, often happen in winter. While most people get measles during
5 the spring and summer.

6 When travelling by plane you should often cross many time zones. During
7 such trips your internal clocks don’t seem to work always properly. We
8 call this jet lag. It makes you feel tired and it may take your body’s
9 biological clock several days to get it used to the new place.
10 People who work night shifts can also have problems with their
11 biological clocks. In general, they may not be as alert or active as such
12 people who work during the daytime. They also have more accidents
13 during the work. In some cases, they experience more health and
14 sleeping problems than other people.

15 By using the right medicine you can even fight off problems that are
16 connected with different times of day. When heart attacks and strokes
17 often happen in the morning– between the time you may get up
18 and noon. Asthma often just occurs between midnight and the morning
19 hours. So when people with weak a heart disease take their medicine
20 right after waking up it might prevent a heart attack.

21 Biological clocks control over many rhythms of life. We are constantly
22 learning more and even more about these rhythms. Doctors are looking
23 for new ways to make travelling more than comfortable and medical
24 treatment more effective.