"You Need Eight Glasses of Water a Day"...And Other Rules to Blow Off

Drink eight glasses of water a day
Forget it! In 2002 Dartmouth Medical School professor Heinz Valtin, MD, published an article in the American Journal of Physiology boldly declaring that, given all the water we get in food and various beverages, simply drinking when thirsty is more than enough for healthy adults who aren't engaging in vigorous exercise. Last year, the nonprofit Institute of Medicine concurred, saying that the "vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide."

Stay out of the sun  
Not entirely. Whoa—it seems that in our eagerness to avoid skin cancer, we went too far down that shady road. According to last June's Harvard Health Letter, allowing sunlight to touch your skin (experts generally suggest ten to 15 minutes a day, twice a week) provides a number of benefits. UVB rays apparently trigger a chemical response that allows your body to produce vitamin D. Some researchers believe this chemical response may even help ward off multiple sclerosis and certain cancers. And people who suffer from seasonal affective disorder (depression caused by lack of light) find significant relief from regular sun exposure.

You need eight hours of sleep a night!
No, you don't. Two studies published in the journal Sleep in 2004 found that people who slept seven rather than eight hours lived longer than their seemingly more well-rested counterparts. That's not to say that sleep deprivation isn't real—get less than seven hours, experts say, and you may begin to lose creativity and focus.