US researchers studied the brain activity of rats kept awake during the day - the time they would usually sleep.

美国的研究人员进行了一项实验，他们让一种习惯昼伏夜出的小老鼠白天不睡觉，以此来研究它们大脑的活动情况。

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A weasellike, usually albino mammal(Mustela putorius furo)related to the polecat and often trained to hunt rats or rabbits.

白鼬，雪貂一种象鼬的通常为白化体的与艾鼬有关的哺乳动物（鼬属雪貂），经常被驯养来猎取鼠和野兔。

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However, malignancies usually occur when fragments of these tumors are transplanted in other rats.

但如把此肿瘤的组织块移植于其他大鼠则经常表现出恶性。

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Rats with tumours were also less eager to drink sugar water, a substance that usually attracts the appetites of healthy rats.

BBC: Tumour make-up 'fuels depression'