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So why do we sleep? This is a question that __________ scientists for centuries

and the answer is, no one  __________ really sure. 

Some believe that sleep gives the body a chance _________ from the day’s activities but in reality, the amount of energy saved by sleeping for even eight hours is minimal, the same amount of energy in a piece of toast.

We  __________ to sleep because it is essential to maintaining normal levels of cognitive skills like speech, memory, innovative and flexible thinking

 In __________ words, sleep plays a significant role in brain development.