Reading in dim light won't damage your eyes, you don't need eight glasses of water a day to stay healthy and shaving your legs won't make the hair grow back faster.
These well-worn theories are among seven "medical myths" exposed in a paper published in the British Medical Journal, which traditionally carries light-hearted features in its Christmas edition.
The research was conducted by Aaron Carroll, an assistant professor of pediatrics at the Regenstrief Institute, Indianapolis, and Rachel Vreeman, fellow in children's health services research at Indiana University School of Medicine.
They took seven common beliefs and searched the archives for evidence to support them.
Despite frequent mentions in the popular press of the need to drink eight glasses of water, they found no scientific basis for the claim.
The complete lack of evidence has been recorded in a study published the American Journal of Psychology, they said.
The other six "myths" are:
Reading in dim light ruins your eyesight
The majority of eye experts believed it was unlikely to do any permanent damage, but it might make you squint, blink more and have trouble focusing, the researchers said.
Shaving makes hair grow back faster or coarser
It has no effect on the thickness or rate of hair regrowth, studies say. But stubble lacks the finer taper of unshaven hair, giving the impression of coarseness.
Eating turkey makes you drowsy
It does contain an amino acid called tryptophan that is involved in sleep and mood control.
But turkey has no more of the acid than chicken or minced beef. Eating lots of food and drink at Christmas are probably the real cause of sleepiness.
We use only 10 percent of our brains
This myth arose as early as 1907 but imaging shows no area of the brain is silent or completely inactive.
Hair and fingernails continue to grow after death
This idea may stem from ghoulish novels. The researchers said the skin dries out and retracts after death, giving the appearance of longer hair or nails.
Cell phones are dangerous in hospitals
Despite widespread concerns, studies have found minimal interference with medical equipment.