Sleep is one of the richest topics in science today: why we need it, why it can be hard to get, and how that affects everything from our athletic performance to our income. Daniel Kripke, co-director of research at the Scripps Clinic Sleep Center in La Jolla, Calif., has looked at the most important question of all. In 2002, he compared death rates among more than 1 million American adults who, as part of a study on cancer prevention, reported their average nightly amount of sleep. To many, his results were surprising, but they’ve since been corroborated by similar studies in Europe and East Asia. Kripke explains.

Q: How much sleep is ideal?

A: Studies show that people who sleep between 6.5 hr. and 7.5 hr. a night, as they report, live the longest. And people who sleep 8 hr. or more, or less than 6.5 hr., they don’t live quite as long. There is just as much risk associated with sleeping too long as with sleeping too short. The big surprise is that long sleep seems to start at 8 hr. Sleeping 8.5 hr. might really be a little worse than sleeping 5 hr.

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