Dec. 21 (HealthDay News) -- Somewhere in the back of your mind is the idea that you should drink at least eight glasses of water a day to stay healthy.

You may have nodded in agreement when someone mentioned scientific studies showing that, on average, we use just 10 percent of our brain.

And you may have lectured your children about the danger of reading in dim light, which could cause permanent eye damage.

None of this is true. But the ideas continue to circulate (and be believed even by some physicians), say Drs. Rachel Vreeman and Aaron Carroll of the Indiana University School of Medicine. They've taken the time and trouble in a two-page report in the Dec. 22-29 British Medical Journal to puncture seven such medical myths.

"We picked these in particular, because we either heard physicians repeat them or heard them in the media a number of times," said Vreeman, who is a fellow in children's health services research at Indiana. "They do appear to be ingrained in the popular imagination, including that of physicians."

Dr. Graham F. Greene, associate professor of urology at the University of Arkansas for Medical Sciences College of Medicine, admits he was tempted to believe the 10 percent brain myth, especially because he'd heard it attributed to Albert Einstein (not so), even though Greene is involved with the UAMS Web page devoted to puncturing similar myths.

"But, in reality, the brain is still a huge mystery in terms of capacity," Greene said. "We're still learning about it."

Greene does recall hearing his mother caution his sisters about another myth punctured in the BMJ presentation -- that shaved hair will grow back coarser and darker. The myth persists even though controlled studies done as long ago as 1928 showed no difference in the growth and texture of shaved and non-shaved hair.

Then, there's the one about hair and fingernails continuing to grow after death -- the so-called Dracula effect. What actually happens, Vreeman and Carroll write, is that the skin retracts after death, giving the illusion of growth. That's part of something that happens during life, too -- we grow "long in the tooth," not because old teeth are growing, but because the gums that support them shrink.

There's an air of scientific verisimilitude about another myth cited by Vreeman and Carroll -- that eating turkey makes you sleepy. Supposedly, that happens because turkey is rich in the sleep-inducing amino acid tryptophan, but the tryptophan content is not great enough to bother anyone, Vreeland said. It's probably the wine that comes with the Thanksgiving turkey that lowers the eyelids.

As for drinking eight glasses of water a day, don't try it. Water comes into the body via a number of foods, such as fruits and vegetables, and a zealous endeavor to meet the eight-glass quota might even be dangerous, Vreeman said.

She and Carroll are expanding their myth-busting effort. "We're in the process of writing a book with over 100 of them," Vreeman said.

Have you heard the one about chewing gum staying in your stomach for seven years if you swallow it?

SOURCES: Rachel Vreeman, fellow, children's health services research, Indiana University School of Medicine, Indianapolis; Graham F. Greene, M.D., associate professor of urology, University of Arkansas for Medical Sciences College of Medicine, Little Rock; Dec. 22-29, 2007, British Medical Journal