Identify the topic sentence in the given paragraph.

Humans need sleep to help their bralns function properly. The brain needs a minimum number of hours to rest and organize itself. The number of hours depends on age. For example, babies need an average of 14 to 16 hours of rest daily, while adults can function on about 6 to 8 hours. Scientists are not completely sure what the brain does during sleep periods, but they do know that the brain sorts and stores information and replaces chemicals when a person sleeps.

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