Half of your brain stays alert and prepared for danger when you sleep in a new place, a study has revealed. This phenomenon is often 26_____ to as the “first-night-effect”. Researchers from Brown University found that a network in the left hemisphere of the brain “remained more active” than the network in the right side of the brain. Playing sounds into the right ears （stimulating the left hemisphere） of 27_____ was more likely to wake them up than if the noises were played into their left ear.

    It was 28_____ observed that the brain was more active during deep sleep. When the researchers repeated the laboratory experiment on the second and third nights they found the left hemisphere could not be stimulated in the same way during deep sleep. The researchers explained that the study demonstrated “when we are in a 29_____ environment, the brain partly remains alert so that humans can defend themselves against any 30_____ danger.”

    The researchers believe this is the first time that the “first-night-effect” of different brain states has been 31_____ in humans. It isn’t, however, the first time it has ever been seen. Some animal 32_____ also display this phenomenon. For example, dolphins, as well as other 33_____ animals shut down one hemisphere of the brain when they go to sleep. A previous study noted that dolphins always 34_____ control their breathing. Without keeping the brain active while sleeping, they would probably down. But, as the human study suggest, another reason for dolphins keeping their eyes open during sleep is that they can look out for 35_____ while asleep. It also keeps their physiological processes working. 

26. referred

解析：动词辨析题。空格前是 is often，空格后是to，空格处应填入动词过去分词。备选项有classified（把……分类），identified（识别），referred（提到、谈到）。根据固定搭配原则，空格处应填referred。

27. volunteers

解析：名词辨析题。空格前是of，空格处应填名词。备选项有novel（小说），potential（潜力），predators（食肉动物），species（物种），varieties（多样性），volunteers（志愿者）。根据语义，实验对象应该是志愿参与者的右耳，故空白处应填volunteers。

28. specifically

解析：副词辨析题。空格前是was，空格后是observed，可知空格处应填入副词。备选项有consciously（有意识地），dramatically（显著地），specifically（特定地、专门地）。再看原文，大脑在深度睡眠状态下更加活跃，这是特别观察的结果，因此应填入specifically。

29. novel

解析：形容词辨析题。空格前是a，空格后是environment，故此处应填入形容词。备选项有exotic（异国风情的），inherent（内在的），marine（海洋的），novel（新颖的），potential（潜在的）。再看原文，大脑的一部分会保持警惕，是因为处于陌生环境，因此应填入novel。

30. potential

解析：形容词辨析题。空格前是any，空格后是danger，空格处应填入形容词。备选项有exotic（异国风情的），inherent（内在的），marine（海洋的），potential（潜在的）。再看空格所在的句子，大脑保持警惕就可以在面对潜在危险情况时保护自己，因此应填入potential。

31. identified

解析：动词辨析题。空格前是has been，空格后是in，因此空格处应填动词过去分词。备选项有classified（把……分类），identified（识别）。根据语义，这是大脑不同区域的首夜效应第一次被识别，因此应填E）identified。

32. species

解析：名词辨析题。空格位于animal之后，可见此处应填入名词。备选项有novel（小说），potential（潜力），predators（食肉动物），species（物种），varieties（多样性）。根据语义，某些动物同样有首夜效应，故正确答案为species。

33. marine

解析：形容词辨析题。空格前other，空格后是animals，空格处应填入形容词。备选项有exotic（异国风情的），inherent（内在的），marine（海洋的）。再看句意，上文出现了海豚，因此应填marine。

34. consciously

解析：副词辨析题。空格前是always，空格后是control，空格处应填入副词。备选项有consciously（有意识地），dramatically（显著地）。根据语义，海豚经常有意识地控制它们的呼吸，此处应填consciously。

35. predators

解析：名词辨析题。空格前是for，因此空格处应填入名词。备选项有novel（小说），potential（潜力），predators（食肉动物），varieties（多样性）。根据语义，海豚睡觉时睁着眼睛是为了提防捕食者，故应填 predators。