Do you really need to drink at least eight glasses of water a day? Does shaving hair cause it to grow back faster or coarser? Does reading in dim light ruin your eyesight?

More: Dieting Myths

The answer to all of those is no, according to a new study in the BMJ, formerly known as the British Medical Journal.

A team of researchers in the U.S. selected seven medical beliefs, espoused by both physicians and members of the general public, for critical review. They then searched for evidence to support or refute each of these claims.

The results showed that all of these medical beliefs range from unproven to untrue.

For example, they found that drinking juice, milk and even caffeinated beverages can give you enough fluid.

The belief that we only use 10 percent of our brains is refuted by studies of patients with brain damage, which suggest that damage to almost any area of the brain affects capabilities. And brain imaging studies also show that no area of the brain is completely silent or inactive.

The belief that hair and fingernails continue to grow after death may be an optical illusion caused by retraction of the skin after death, they added. The actual growth of hair and nails requires a complex interplay of hormonal regulation not present after death.

A trick of the eye could also be to blame for the belief that shaving hair causes it to grow back faster, darker and coarser, they said. The stubble resulting from shaving grows out without the finer taper seen at the ends of unshaven hair, giving the impression of thickness and coarseness.

Finally, expert opinion is that reading in dim light does not damage your eyes, and there is little evidence to support beliefs such as banning mobile phones from hospitals on the basis of electromagnetic interference.

Despite their popularity, all of these medical beliefs range from unproven to untrue, say the authors. They suggest that physicians should constantly evaluate the validity of their knowledge.