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 en6ugh 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 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 sufferer should check the mattress, and bedroom temperature, and make any necessary changes. He should relax for an hour or two before bedtime, perhaps by reading a book, watching television or taking a warm bath. If he wakes during the night he should try turning on the light and reading for a while. Taking warm milk and a biscuit or two may be helpful.

According to the writer, the most serious aspect of insomnia is generally ______.