WASHINGTON — Just because your doctor tells you to drink eight glasses of water a day doesn't mean you should, according to researchers at the Indiana University School of Medicine.

Doctors often fall for the same health myths that their patients do, Drs. Rachel Vreeman and Aaron Carroll report in the Christmas-New Year's issue of the British Medical Journal. Among seven myths they cite is the eight-glasses-of-water one.

"There is no medical evidence to suggest that you need that much water," Vreeman concluded after a long review of research on the subject.

She and Carroll trace the misperception to a 1945 recommendation by the Nutrition Council that Americans consume the equivalent of eight glasses of fluids daily. Lost over the years, they concluded, was the council's note that the 64 ounces called for included water contained in coffee, soda, fruits and vegetables.

Based on informal polls of their colleagues at the Indianapolis-based medical school, other unjustified convictions included:

n People use only 10 percent of their brains. Lots of evidence refutes that common belief, they say. Most compelling is the finding that damage to almost any area of the brain has powerful effects on thinking and behavior.

n Hair and fingernails keep growing after death. According to forensic anthropologist William Maples, whom the researchers cite: "It's a powerful, disturbing image, but it is pure moonshine." What does happen, Maples found, is that dehydration after death sometimes causes skin around the hair or nails to shrink.

Vreeman and Carroll also take on claims, endorsed by some doctors they quizzed, that reading by dim light ruins eyesight; eating turkey makes you sleepy; shaved hair grows back faster, darker or coarser; and cell phones seriously disrupt hospital electronics.

In each case, Carroll said, "we're not saying that it's a lie, but that at best nobody knows. At worst, it's not true."

In the nobody-really-knows category, they said, are the effects of dim light on vision and the extent to which cell phones might interfere with medical devices.

The pair's debunking research got started, Carroll said, after he heard a doctor caution in a pre-Halloween radio interview against strangers poisoning kids with candy.

"I knew that there was no documented case of that, and I thought that a doctor shouldn't be raising the fear," Carroll said.

From raising the matter with other doctors, they found that everyone "believed at least one thing was true that we knew from our list (of seven beliefs above) was untrue or unproven."