8 hours -- or else

Americans are sleeping less than ever, according to a new National Sleep Foundation poll. Some people are losing sleep because of the economy. Some are staying up too late and getting up too early. Some have disorders such as sleep apnea. And others can't sleep because their partner snores. But the danger is the same for everybody, says Mark Opp, a professor at the University of Michigan and the senior writer of a study about sleep in this month's Nature Reviews Neuroscience.

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