From Newsweek's top ten health myths, two of my least-favorite:
Reading in dim light ruins your eyesight. While this is one myth that parents around the world have loved for generations, it has very little scientific backing. Reading in the dark can cause a temporary strain on the eyes, but it rapidly goes away once you return to bright light. The practice has been blamed for increasing rates of myopia (nearsightedness), but Carroll says those claims don't align with the evidence—we're living in the best-lit conditions the world has ever seen. "Seventy years ago we were reading by candlelight and weren't going blind," says Carroll. "There's no evidence for this whatsoever."
You should drink at least eight glasses of water a day. The source for this myth may be a 1945 article from the National Research Council that claims that a "suitable allowance" of water for adults is 2.5 liters a day, although the last sentence of the article notes that much of that water is already contained in the food we eat. Existing studies suggest that that often-omitted fact is key to understanding water intake. We get enough fluids from our typical daily consumption of juice, milk and even caffeinated drinks. And drinking too much water can cause water intoxication, a severe electrolyte imbalance in which cells swell with excess fluid, and even death.
And, as an extra bonus, this one:
We use only 10 percent of our brains. The notion that our brains are not running at full speed simply doesn't hold up. "Numerous types of brain imaging studies show that no area of the brain is completely silent or inactive. Detailed probing of the brain has failed to identify the 'nonfunctioning' 90 percent," Carroll and Rachel Vreeman, an assistant professor of pediatrics at the Indiana University School of Medicine, write in the British Medical Journal study. Carroll says the notion may go as far back as the snake-oil salesmen of the early 20th century, who used the myth to sell a tonic that would increase brainpower.