While humans sometimes cannot sleep when tired, there is no evidence that they can sleep when they are not tired, Klerman explained.
科勒曼说，而人有时在太累的情况下睡不着，却没有证据证明他们在不累的时候可以睡着。
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Younger subjects slept for an average of 9 hours compared to 7.5 for older people, said Klerman and her colleague Derk-Jan Dijk of the Surrey Sleep Research Centre in England.