Eight glasses of water and other medical myths.
For the last few years we have been bombarded with advice to drink more water - up to eight glasses a day. And we've also been told not to read in poor light as it will affect our eyesight and even that shaving hair makes it grow back faster and coarser.
But is there any truth to these and other widely-held beliefs?
A team of researchers from the US set out to discover whether there is any evidence to support seven such assumptions, including the big water question.
They found that most are either unproven or untrue, and should be regarded not as universal truths, but more likely as medical myths.
The authors of the research said, "The medical myths we give here are a light-hearted reminder that we can be wrong and need to question what other falsehoods we unwittingly propagate as we practise medicine."
The eight glasses of water rule centres on the belief that slight dehydration leads to a big drop in energy levels.
But despite the raft of recommendations for drinking eight a day, the research team from the Indiana University School of Medicine could find no proven backing for knocking back such a large volume.
Instead studies have suggested that adequate fluid intake can be met by drinking juice, milk, and even caffeinated beverages, such as coffee and tea.
Clinical evidence actually pointed to the dangers of drinking excessive amounts of water, the authors said.
The belief that we use only 10% of our brains was also refuted by studies of patients with brain damage, which suggested damage to almost any area of the brain had specific and lasting effects on mental, vegetative and behavioural capabilities.
Brain imaging studies have also shown that no area of the brain is completely silent or inactive, the authors said in the Christmas edition of the British Medical Journal, which is published today.
Another belief - that hair and fingernails continue to grow after death - was also challenged by the authors.
They said it may be an illusion caused by retraction of the skin after death.
"Dehydration of the body after death and drying or desiccation may lead to retraction of the skin around the hair or nails.
"The skin's retraction can create an appearance of increased length or of greater prominence because of the contrast between the shrunken soft tissues and the nails or hair.
"The actual growth of hair and nails, however, requires a complex hormonal regulation not sustained after death," they said.
And an illusion may be responsible for the belief that shaving hair causes it to grow back faster, darker and coarser, the researchers said.
Stubble grows without the finer taper seen at the ends of unshaven hair, giving the impression of thickness and coarseness. But its growth rate or type of growth is actually unaffected.
"Similarly, the new hair has not yet been lightened by the sun or other chemical exposures, resulting in an appearance that seems darker than existing hair," they said.
The authors also found no expert evidence to support the belief - usually spread by worried parents - that reading in dim light damages eyesight.
Likewise, there was little evidence to support banning mobile phones from hospitals on the basis of electromagnetic interference.
Finally, and particularly relevant at Christmas time was the belief that eating turkey makes people sleepy - a key excuse for that nap next Tuesday afternoon!
This was thought to be a result of the tryptophan amino acid it contains.
But the authors said, "Actually, turkey does not contain an exceptional amount of tryptophan. Turkey, chicken and minced beef contain similar amounts of tryptophan (about 350mg per 115g), and other sources of protein, such as pork or cheese, contain more tryptophan per gram than turkey.
"The effects of tryptophan in turkey are probably minimised by consuming it with other food, which may limit its absorption.
"Any large meal (such as turkey, sausages, stuffing and vegetables, followed by Christmas pudding and brandy butter) can induce sleepiness because blood flow and oxygenation to the brain decrease, and meals rich in protein or carbohydrate may cause drowsiness. Wine may also play a role.": Stay away from the disco biscuits or you may end up as a Hasselhoff:Ever wondered what a "disco biscuit" is, or why doctors in accident and emergency departments may be referring to you as a "Hasselhoff", rather than using your real name? These are the latest examples of medical slang, used as shorthand by a growing number of medics practising in our hospitals. Dr Paul Keeley, a consultant in palliative medicine at Glasgow Royal Infirmary, has compiled a list of some of the newest slang terms to enter the medical lexicon, which has been published in the British Medical Journal. These include: 404 moment - the point in a ward round when, despite searches of the notes or electronic records, a result cannot be found. This originates from the world wide web error message "404 document not found";
Disco biscuits - otherwise known as the class A drug, ecstasy. The phrase might be used by an A&E doctor, for example, "The man in cubicle three looks like he's taken one too many disco biscuits";
MacTilt - the tilting of the head by a Macmillan nurse, a specialist palliative care nurse. It is intended to convey sympathy and understanding;
Hasselhoff - an injury with a bizarre explanation presenting to accident and emergency. Named after the former Baywatch actor David Hasselhoff, below, who suffered a freak injury in 2006 when he hit his head on a chandelier while shaving. The broken glass severed four tendons and an artery in his right arm; Jack Bauer - a doctor still up and working after 24 hours on the job. This is from the lead character in the television series 24; Mini Me - a trainee or medical student who emulates their senior colleague a little too much, but doesn't say a lot. The name comes from the Austin Powers films; Blamestorming - a session of mutual recrimination during which a multidisciplinary team attempts to apportion blame for a glaring error; Ringo - an expendable member of the team, named after Beatles drummer Ringo Starr. While the other members of the band all enjoyed successful solo music careers, Ringo did the voiceover for Thomas the Tank Engine; Fonzie - a middle grade doctor seemingly unflappable in any medical emergency. Based on the character Arthur "Fonzie" Fonzarelli from the American sitcom Happy Days.