Sleep need is partly genetic and may be determined by other factors that also influence life span, he says.

睡眠的需求一部分取决于基因，也可能取决于其他一些因素，这些因素还影响生命的长短。

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Yet sleep scientist Ying-Hui Fu at the University of California, San Francisco,says little is known about the basic need for sleep.

VOA: special.2009.08.26

Studies have shown that over time, most humans need eight to eight and a half hours of sleep for the best health.

VOA: special.2009.08.26

But here is something that scientists now know: A team led by Professor Fu has reported the first genetic link to how much sleep we need.

VOA: special.2009.08.26

After many years of study, no one knows how much sleep we need to function normally.

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The important thing, says Scharf, is that people who need sleep aids can take them.

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Don't start reading this at bedtime if you need sleep before going to work.

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