In 99 cases out of 100, insomnia(失眠) is caused by a disturbance of the natural sleep rhythm. The reasons why the rhythm has been disturbed are many and they range from drugs that are being taken to treat a separate medical condition to anxiety. A sudden change in lifestyle. or climate could do it, or just that you’ve fallen into the habit of dozing off(打瞌睡) in front of the television.

 Certainly the body must have enough sleep. Tests that deprived people of sleep have proved lack of it can cause fairly rapid physical and mental deterioration(恶化). But on the other hand, it doesn’t need too much. So if you’re sleeping in front of the TV, you won’t 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.

 Of course, the problem for my summer insomniacs is that, despite an almost overwhelming urge to put their heads down on their desks in the afternoons, they aren’t getting any extra sleep to compensate for their wakeful nights. For the worst sufferers I sometimes prescribe(开药方) 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 ______.