阅读下面短文，用括号内所给动词的适当形式填空(必更时可加情态动词或助动词)。
If you've ever stayed up late and then gotten up early the next day, you know what it's like not to get enough sleep. It's very uncomfortable feeling. In fact, for many years scientists 44．(believe) that sleep is like food - if we don’t get enough of it, we might get sick or even die. But now they 45． (start)to wonder just how much sleep we need.
Researchers at Imperial College London did a study on fruit flies(果蝇). They found that flies that slept for very short periods of time - about72 minutes a day -still 46． (live) about as long as flies that slept for normal amounts of time. One of the female slept for just 4 minutes each day on average. This has led the researchers to wonder if sleep is less necessary than we previously thought.
Humans are very different from fruit flies. Sleep 47．(help) repair tissue (组织) when our bodies perform a lot of maintenance (维护). Sleeping allows our bodies 48． (get) well again from the stress we experience each day. But can we really die from not sleeping? No humans have died so far from 49． (stay) awake for too long. The current(现在的)world record for sleeplessness 50． (hold) by Randy Gardner from the US, who stayed awake for 11 days and 25 minutes in 1964.
Scientist have not yet found any animals that don’t sleep at all. Even animals that only sleep for short periods are not completely sleepless.
44．believe
45．are starting
46．lived
47．helps
48．to get
49．staying
50．was held
【解析】
本文介绍了没有足够的睡眠人会感觉很不舒服。多年来，科学家们一直认为睡眠就像食物——如果我们睡眠不足，我们可能会生病甚至死亡。现在他们开始想知道我们到底需要多少睡眠。伦敦帝国理工学院的研究人员对果蝇进行了研究。这使得研究人员怀疑睡眠是否比我们之前想象的要少。到目前为止，还没有人死于长时间的不睡觉。美国兰迪·加德纳（Randy Gardner）的《目前的失眠世界纪录》（The current）于1964年保持清醒11天25分钟。科学家还没有发现任何不睡觉的动物。即使只睡很短时间的动物也不会完全失眠。
44．句意：事实上，多年来，科学家们认为睡眠就像食物——如果我们睡眠不足，我们可能会生病甚至死亡。
根据that sleep is like food - if we don’t get enough of it, we might get sick or even die. 睡眠就像食物——如果我们睡眠不足，我们可能会生病甚至死亡。可知，这种情况是科学家们多年来一直认为的一种情况，因此应该用一般现在时；主语scientists是复数，因此谓语动词用原形。故答案为：believe。
45．句意：但现在他们开始想知道我们到底需要多少睡眠。
由句中的now可知，该句话的时态应该用现在进行时，其结构是：am/is/are+动词的现在分词。主语是they，因此be动词用are，故答案为：are starting。
46．句意：他们发现，睡眠时间很短的苍蝇——每天大约72分钟——仍然和正常睡眠时间差不多长。
根据They found，可知，主句的时态为一般过去时，因此在由that引导的宾语从句that flies （that slept for very short periods of time - about72 minutes a day -）still ______3______ (live) about as long as flies that slept for normal amounts of time. 也要用一般过去时，故答案为：lived。（其中的句子that slept for very short periods of time是定语从句，修饰flies；- about72 minutes a day -：用来解释说明very short periods of time的。）
47．句意：当我们的身体进行大量的保养时，睡眠有助于修复组织。
根据Humans are very different from fruit flies. 人类和果蝇有很大的不同。可知，下文应该是介绍不同的原因，也就是介绍人类睡眠的作用，因此应该用一般现在时。主语Sleep是单数，所以谓语用三单形式。故答案为：helps。
48．句意：睡眠可以让我们的身体从我们每天经历的压力中恢复。
本题考查：allow sb. to do sth. 允许某人做某事。因此后面应该用动词不定式做宾补。故答案为：to get。
49．句意：到目前为止，还没有人死于长时间的不睡觉。
die from：死于（某种原因，不包括疾病、过度悲伤等）；from是介词，后跟动名词，stay的动名词为staying，stay awake for too long：保持清醒很长时间；故答案为：staying。
50．句意：美国的兰迪·加德纳（Randy Gardner）保持了目前的不睡觉世界纪录，他在1964年保持了11天25分钟的清醒。
分析句子结构可知，主语The current(现在的)world record for sleeplessness与动词hold之间存在着被动关系，因此应该用被动句，而且由时间in 1964可知，该句话应该用一般过去时，所以该句话是一般过去时的被动句。其结构是：was/were+动词的过去分词。主语The current(现在的)world record for sleeplessness是单数，因此be动词用was。故答案为：was held。
