Dallas ? In the largest study of sleep habits to date, researchers reported Thursday that Americans appear to live longer when they average five to seven hours of sleep a night.

As a group, people who slept moderate amounts were least likely to have died in the six-year period in which the study was conducted. People who slept more than eight hours a night, or less than four hours, showed an increased risk of death, although the study could not explain why.

“We’re just saying it’s safe to sleep five, six or seven hours a night,” said Dr. Daniel Kripke, a psychiatrist at the University of California, San Diego and the study’s lead researcher.

The study, based on a survey of 1.1 million adults, found that a slim majority 51 percent of the women and 52 percent of men already are sleeping five to seven hours a night. Those who sleep eight hours include 39 percent of the women and 38 percent of the men.

The findings appear to shatter the long-held belief that people need eight hours of sleep to maintain a healthy lifestyle. Women who slept eight hours, in fact, had a 13 percent greater risk of dying in six years, while men had a 12 percent increased risk of death, the study found.

“We don’t know that longer sleep is causing that risk,” Dr. Kripke said. The researchers took into account 30 health factors, such as diet, exercise and disease history, and found that these did not explain why people with different sleep habits had different death rates.

Based on the study, “I also wouldn’t advise people to change their sleeping duration,” Dr. Kripke says.

The study’s conclusions may cause other experts to lose sleep, however.

The National Sleep Foundation issued an immediate and scathing analysis of the study’s findings, which were published in the Archives of General Psychiatry, a scientific journal of the American Medical Assn.