1. To function best, you need to get eight hours.
There's nothing magic about that number. Everyone has different sleep needs, and you'll know you're getting enough when you don't feel like nodding off in a boring situation in the afternoon, says New York University psychologist Joyce Walsleben, Ph.D., co-author of A Woman's Guide to Sleep.

2. If you can get it, more sleep is always healthier.
You wish. Some studies have found that people who slept more than eight hours a night died younger than people who got between six and eight hours. What scientists don't know yet: Whether sleeping longer causes poor health or is a symptom of it, says Najib Ayas, M.D., MPH, assistant professor of medicine at the University of British Columbia. Long sleepers may suffer from problems such as sleep apnea, depression, or uncontrolled diabetes that make them spend more time in bed.

3. Some people function perfectly on four hours of sleep.
Legendary short sleepers—including Bill Clinton, Madonna, and Margaret Thatcher—don't necessarily do better on fewer Zs. "They're just not aware of how sleepy they are," says Thomas Roth, Ph.D., sleep researcher at Henry Ford Hospital in Detroit. Too little sleep is bad for your health and your image: It can make you ineffective (it impairs performance, judgment, and the ability to pay attention), sick (it weakens your immune system), and overweight. In fact, women who slept five hours or less a night were a third more likely to gain 33 pounds or more over 16 years than women who slept seven hours, according to a Harvard Nurses' Health Study. Oddly, cutting too much sleep and getting less than six hours is associated with the same problems as sleeping too long: a higher risk of heart problems and death. And, of course, cheating on sleep hurts you behind the wheel: "Wakefulness for 18 hours makes you perform almost as though you're legally drunk," says Walsleben.

antihistamines do the same.

4. Waking up during the night means you'll be tired all day.
Au contraire: It might be our natural cycle. Many animals sleep this way, and there are a lot of indications that our ancestors did, too, perhaps stirring nightly to talk or have sex, says Thomas Wehr, M.D., scientist emeritus at the National Institute of Mental Health. When 15 people in one of his studies lived without artificial lights for a few weeks, they wound up sleeping three to five hours, waking up for one or two, then sleeping again for four or more hours—and they said they had never felt so rested.

5. You need prescription drugs if you have insomnia every night.
Sleep meds are designed for short-term sleep problems, caused by stressful events like the loss of a job or taking a transatlantic flight. People with longer-term problems benefit more from cognitive behavioral therapy—essentially, retraining your perceptions of sleep and learning better sleep habits, such as going to bed at the same time every night, avoiding TVs and computers before bed, staying away from caffeine at least six hours before sleep, and other lifestyle changes. In fact, in 2005, the National Institutes of Health concluded that this type of therapy is as effective as prescription drugs for short-term treatment of chronic insomnia. In many cases, a sleeping pill may not even solve your sleep problem. "About half the people who think they have insomnia may have anxiety or depression," says Daniel Kripke, M.D., a University of California at San Diego sleep expert.

6. You can make up for lost sleep on weekends.
Bingeing on Zs over the weekend and not sleeping during the week—what Harvard sleep expert Robert Stickgold, Ph.D., calls "sleep bulimia"—upsets your circadian rhythms and makes it even harder to get refreshing sleep. Sleeping until noon on Sunday generally prevents you from hitting the sheets by 10 that night. So instead of correcting your deficit from the week before, you set up a no-sleep cycle for the week to come. "The body loves consistency," says Donna Arand, Ph.D., spokeswoman for the American Academy of Sleep Medicine. Best to rise around the same time every day, even on weekends.

7. Tylenol PM is better than a prescription sleep med for an occasional bout of insomnia.
Not if the bout lasts longer than a few nights, says Helene Emsellem, M.D., of the Center for Sleep & Wake Disorders in Chevy Chase, MD. Tylenol PM is no better than a prescription drug for people who have trouble falling asleep, and may be less effective than some prescription drugs, she says. The active ingredient in Tylenol PM is an anti-histamine, and its side effect is that it makes you drowsy. Some have reported a greater possibility of feeling "hung-over" after taking antihistamines than after taking prescription drugs. If you do decide to take antihistamines, don't do it in the middle of the night: They may stay active in your system for eight hours or more.

What Do You Think?