Nope, 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 Joyce Walsleben, Ph.D., of New York University School of Medicine and co-author of A Woman’s Guide to Sleep.

2. If you can get it, more sleep is always healthier.

On the contrary. Some studies have found that people who slept more than eight hours a night died younger than people who snoozed 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., of the University of British Columbia. Long sleepers may actually 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 former President Bill Clinton, Madonna and Margaret Thatcher, don’t necessarily do better with less sleep. “They just may not be aware of how sleepy they are,” says Thomas Roth, Ph.D., of the Sleep Disorders and Research Center 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 lbs. or more over 16 years than women who slept seven hours, according to the Harvard Nurses’ Health Study.

Oddly, getting less than six hours is associated with the same problems as sleeping too long — a higher risk of heart problems and death. Plus, cheating on sleep hurts you behind the wheel: “Wakefulness for 18 hours makes you perform almost as though you’re legally drunk,” says Walsleben. Yikes!

4. Waking up during the night means you’ll be tired all day.

Not necessarily. It might be our natural cycle. Many animals sleep this way, and there are many indications that our ancestors did, too, perhaps stirring nightly to talk or mate, 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 and then sleeping again for four or more hours. But — here comes the surprise — 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 that are caused by stressful events like the loss of a job or traveling on a transatlantic flight. People with longer-term problems benefit more from retraining themselves, such as designating a “worry time” in the afternoon so they learn not to associate bedtime with stress, going to bed at the same time every night, avoiding TVs and computers before bed and staying away from caffeine at least six hours before sleep.

In fact, in 2005, the National Institutes of Health concluded that this type of training is as effective as prescription drugs for short-term treatment of chronic insomnia — head to a local university’s sleep center or find an accredited sleep center from the American Academy of Sleep Medicine at sleepcenters.org.

And in many cases, a sleeping pill might not even solve your sleep problem. “About half the people who think they have insomnia may have anxiety or depression,” says sleep expert Daniel Kripke, M.D., of the University of California at San Diego. If you’ve been feeling anxious or down in the dumps for several weeks or more, get your doctor or a friend to recommend a therapist.

6. You can make up for lost sleep on weekends.

Bingeing on pillow time 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 slumber. Sleeping until noon on Sunday generally prevents you from hitting the hay by 10 that night.

So instead of correcting your lost hours 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., spokesperson 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. One of the active ingredients in Tylenol PM is an antihistamine, and its side effect is that it makes you drowsy. Some have even reported a greater possibility of feeling “hungover” after taking antihistamines than after taking prescription drugs.

If you do decide to take antihistamines, don’t pop one in the middle of the night: They may stay active in your system for eight hours or more. Ask your doctor about prescription pills if you find yourself using over-the-counter meds for more than two weeks, or if you’re not seeing results. And for best results, learn to change your sleep attitudes and behavior — both types of sleep meds can be addictive long-term.