In 99 cases out of a 100, insomnia(失眠) is caused by a disturbance of the natural sleep rhythm. The reasons why the rhythm has been disturbed are many they can range from drugs to alcohol to mental illness to health problems to unresolved stress to bad sleep habits. A sudden change in lifestyle. or climate can do it, or a change in a job or an emotional relationship. Certainly the body must have enough sleep. Tests of sleep-deprived people have proved that lack of sleep can cause fairly rapid physical and mental deterioration(退化). On the other hand, the body doesn't need too much sleep. So if you're sleeping in front of the TV, you might find it difficult to sleep soundly at night. Similarly, if you're holidaying in Spain and spending your days sleeping on the beach, chances are that you'll be wide awake at bedtime. One interesting phenomenon regarding insomnia is that of summer insomnia. Some people have difficulty sleeping during the summer due to longer daylight hours and warmer nights. Many summer insomniacs feel an almost overwhelming urge to put their heads down on their desks in the afternoons, but they can't find a way to compensate for their wakeful nights. For the worst sufferers I sometimes prescribe a mild sleeping pill which, after a few nights, reestablishes the body's natural sleep rhythm, conditioning it to accept the heat. From the passage, insomnia results from ______.