Doctors pour cold water on commonly held viewsAnd they use more than 10% of their brains to do itWhat other medical myths are out there? Tell us about them here

Heard the one about reading in dim light being bad for your eyes? It's just a myth

They are the universal pearls of wisdom that explain some of the more puzzling things about the human body and help people live healthy lives: don't read in dim light, drink eight glasses of water a day and don't use mobile phones in hospitals.

The problem is: there is no evidence to suggest that these gems of advice are actually true. In a study out today researchers have scoured through leading databases of medical research to test whether any of the most commonly held beliefs among doctors and patients bear any links to reality.

The two doctors behind the research wanted to remind their colleagues that anyone could get things wrong and suggested that doctors should think twice about commonly held ideas that might not be based on evidence.

"We got fired up about this because we knew that physicians accepted these beliefs and were passing this information along to their patients. And these beliefs are frequently cited in the popular media. We didn't set out to become myth busters," said Aaron Carroll of the Regenstrief Institute in Indianapolis.

Along with Rachel Vreeman of the Indiana University School of Medicine, Carroll examined beliefs ranging from healthy living to the morbid idea that nails and hair keep growing after you die.

The seven myths, published today in the British Medical Journal, were based on ideas and conversations the authors had heard endorsed on several occasions - and which many physicians thought were true.

"Whenever we talk about this work, doctors at first express disbelief that these things are not true. But after we carefully lay out medical evidence, they are very willing to accept that these beliefs are actually false," said Vreeman.

Everyone must drink at least eight glasses of water a day

This advice is thought to have originated in 1945 from the Nutrition Council in the US, which suggested people needed to consume 2.5 litres of water a day. But Vreeman said the water contained in food, particularly fruit and vegetables, as well as in milk, juice, coffee and soft drinks, also counts towards the total.

We only use 10% of our brains

"The myth arose as early as 1907, propagated by multiple sources advocating the power of self-improvement and tapping into each person's unrealised latent abilities," say Vreeman and Carroll. "The many functions of the brain are highly localised, with different tasks allocated to different anatomical regions. Detailed probing of the brain has failed to identify the 'non-functioning' 90%."

Hair and fingernails continue to grow after death

According to Vreeman, the myth persists because of an optical illusion. "As the body's skin is drying out, soft tissue, especially skin, is retracting. The nails appear much more prominent as the skin dries out." The same is true, but less obvious, with hair. "As the skin is shrinking back, the hair looks more prominent or sticks up a bit." The actual growth of hair and nails requires a complex set of precisely regulated hormones, which do not continue after death.

Reading in dim light ruins your eyesight

Generations of parents have warned their children not to read in poor light, telling them that it could somehow damage their sight. Though dim lighting can cause stress in the eye, the important thing to remember, say the researchers, is that the effects are not permanent. "Suboptimal lighting can create a sensation of having difficulty in focusing. It also decreases the rate of blinking and leads to discomfort from drying. The important counterpoint is that these effects do not persist."

Shaving causes hair to grow back faster or coarser

This theory may go some way toward sustaining the multimillion-pound waxing industry. But it, too, is an optical illusion: when hair first grows after being shaved, it has a blunt edge on top, which gets worn away over time and results in the fine taper of long hair. Also, the sun naturally bleaches hair over time so hair that is newly emerged may seem darker but is, in fact, no darker than any other new hair growth.

Mobile phones are dangerous in hospitals

Early studies in the UK showed that mobile phones interfered with only 4% of devices and only at a distance of less than one metre. In a 2005 US study, 510 tests were carried out with 16 medical devices and six mobile phones - the incidence of clinically important interference was 1.2%. Conversely, a recent survey of anaesthetists found that the use of mobiles by doctors reduced the risk of medical error or injury from delays in communication between hospital staff.

Eating turkey makes you especially drowsy

Vreeman and Carroll admitted that even they initially believed this myth. Tryptophan, a chemical that is contained in turkey, can cause drowsiness but there isn't really very much of it in the bird. Chicken and ground beef contain about the same amount per gram while other protein sources such as pork and Swiss cheese contain more.

The real reason you might feel sleepy after a huge Christmas dinner is more straightforward: the vast amount of food diverts blood away from the brain and towards the stomach, which has the mammoth task of digesting the turkey, sausages, stuffing, vegetables and Christmas pudding.

What pearls of wisdom do you believe in?

blogs.theguardian.com/science