New Delhi, Aug. 26 -- "EIGHT HOURS labour, eight hours recreation, eight hours rest," said Robert Owen, a Welsh textile-maker, in 1817. His aim was a fairer work week, but the idea that sleeping eight hours a night is best has since become the received wisdom. Is it right?

Not getting enough kip can certainly be a problem. A bad night's sleep can lead to a drop in cognitive abilities, increased stress and general grumpiness the following day. Chronic sleep deprivation is associated with cognitive decline, psychiatric disorders and even an early death. But too much sleep may also be bad for you. The evidence suggests a U-shaped relationship between health and sleep duration, with both insufficient and excessive slumber being worse than an...