Medical myths

Medical myths exposed the Guardian! (I confess, I believed the last one):

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.

Yep, turkey doesn't make you especially drowsy. I feel drowsy after Christmas dinners because I stuffed my face. (I'll try to be more careful this year, honest!)