Samena Chaudhry Birmingham

Excessive sleeping may increase your risk of an early death by up to 15%, according to a new analysis of data collected on one million people by the American Cancer Society. The figures cast doubt on the reputed benefits of eight hours' sleep a night.

Psychiatrists at the University of California, San Diego, found that people with the longest lives slept for only seven hours each night (Archives of General Psychiatry 2002;59:131-6). Why seven is the magic number is not clear. Sleeping more seems to be riskier than sleeping less, although less than four hours a night is also associated with increased mortality.

The researchers, who controlled for other factors such as weight, smoking, and exercise, claim to be the first to determine the relation between sleep and mortality.

"If people don't sleep eight hours, they have nothing to worry about," says Daniel Kripke, a member of the team. Insomniacs, the data suggest, have no greater risk of premature death. Sleeping pills, in contrast, might just take some days off your life. Kripke's group plans to conduct experiments to determine whether setting the alarm clock can lengthen survival.

But critics of the study hope it will not be overinterpreted. They point out that the enormous dataset--part of a cancer prevention study in the 1980s--was not specifically designed to analyse sleep. Insomnia was not defined, sleeping pills were not identified, and participants reported their own sleep behaviour in one hour increments, leaving little room for fine detail.