[单项选择]

Insomnia (失眠) is the inability to sleep calmly and relaxedly. It is a problem to nearly everyone at some time. A person may be awake for an hour in the middle of the night and then, next morning, may feel that he hardly slept all night. Normally, worry about not getting enough sleep is the worst aspect of most insomnia. If insomnia occurs repeatedly, it is important to find its possible causes and try to correct them.
Any of simple factors may make one feel difficult to fall asleep. His mattress (床垫) may be too soft or too hard. If there are too many blankets (毯子) on a hot night, he may be too warm; if the mattress is too light he may feel cold during the night. The bedroom itself may be overheated or too cold. If it is not dark enough, or too noisy, this may cause difficulty in sleeping. Eating shortly before going to bed may also be responsible for insomnia. Drinking tea or coffee can also make people awake.
The cause of insomnia may be easy to correct. The suff
A. the repeated occurrence of insomnia
B. the worry about less time for sleep
C. the difficulty in changing it
D. the tiredness from the insomnia