You don’t need sunscreen if you’ll be indoors all day
It’s smart to wear sunscreen all the time, especially in the car or if you sit near a window. Here’s why: The sun contains UVA and UVB rays, both of which play a role in skin aging, eye damage, and skin cancers, according to the Skin Cancer Foundation. And while UBV rays (which are responsible for giving you a sunburn) don’t penetrate window glass, UVA rays do—and they can also cause skin damage way down in the skin’s epidermis. One 2011 study in the Journal of the American Academy of Dermatology found that 52% of melanoma cancers are found on the left side of the body; the researchers suspect that this is partly because in the United States, drivers’ left sides face the window.
“Just because you don’t have a sunburn doesn’t mean the sun isn’t damaging your skin, says Filamer Kabigting, MD, a dermatologist at ColumbiaDoctors.
Birth control pills need to “clear” from your system before you can get pregnant
“I hear this regularly,” says Dr. Minkin. “It’s been around for probably the last 40 years.” But the truth is, once you stop taking the pills, you can get pregnant because your body is no longer receiving that extra dose of pregnancy-preventing estrogen or progestin hormones, she says.
(Remember, it’s even possible—though very rare—to get pregnant while taking birth control. According to the National Institutes of Health, an estimated 2 or 3 women out of 100 may become pregnant while on the pill—even if they never miss a dose.)
You need to drink 8 glasses of water a day
Experts have long been baffled about where this myth came from. One 2002 paper in the American Journal of Physiology combed the literature and couldn’t find any scientific evidence that backed up this theory. One guess, according to the authors, was a 1945 report written by the Food and Nutrition Board of the National Research Council that said: “A suitable allowance of water for adults is 2.5 liters [about 10 cups] daily in most instances… Most of this quantity is contained in prepared foods.” Emphasis on that last part.
In fact, there’s no rule that you have to get all your fluid intake from water alone, says Marie Spano, RD, a sports nutritionist. You can get plenty from food. “Soups, fruits, and vegetables absolutely count,” she says.
Caffeine dehydrates you
You’ve probably been told that not only will coffee not keep you hydrated, but that since caffeine is a diuretic, you should drink extra water to compensate. But as it turns out, if you’re a habitual coffee drinker, you can go ahead and count that venti Americano to your daily fluid intake. In a 2013 study by researchers from the University of Bath, people who drank moderate amounts of coffee a day (which contained a range of about 200 to 450 milligrams of caffeine) weren’t any more dehydrated at the end of 3 days than those who just drank water. While caffeine itself can act as a diuretic, say the study authors, people likely build up a tolerance, which makes them able to stay hydrated.
Eating turkey makes you sleepy
Experts have known for years that tryptophan—an amino acid found in turkey—can make people feel sleepy. But the thing is, plenty of other foods, like pork, lamb, and beef, contain about as much tryptophan as turkey, and we don’t think those make us sleepy, says Spano. As for why you crashed on your grandma’s couch after Thanksgiving dinner? Blame all the carbs and sugar you just ate. (Immediately sitting down to watch the football game probably didn’t help either.)