Studies show that people who sleep between 6.5 hr. and 7.5 hr. a night live the longest, according to Daniel Kripke, director of research at the Scripps Clinic Sleep Center. "We can prevent a lot of insomnia and distress just by telling people that short sleep is O.K.," said Kripke. "We've all been told you ought to sleep 8 hr., but there was never any evidence."
