Zzzzzzzz! How much sleep is enough?

Q: I usually wake up exhausted, not refreshed, even when I do get enough sleep. Why is this?

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A: It’s not just the quantity of sleep that’s counts, but also the quality. To feel fully rested, your brain must go through five phases of sleep. In addition, you need to spend enough time in each phase. Anything that interrupts these phases contributes to your fatigue.

At least 40 percent of American women sometimes can’t find the time to sleep. They’re simply too busy! And not everyone needs eight hours a night — some people need as many as 10 hours.

Sleep is important, as you know. Lack of sleep makes it hard to function during the day. It’s also a major cause of auto accidents.

Besides the time crunch, factors that contribute to poor sleep include:

Changes in your inner clock
Your body’s circadian rhythm responds to light-dark cycles. When darkness falls, the brain’s pineal gland secretes melatonin, which causes drowsiness.

As we get older, this inner clock changes. For teenagers it is set to “late to bed, late to rise.” After years of sleep experience, we are more likely to fall asleep at dusk and awaken at dawn.

There are many times when your alarm clock and your inner clock simply don’t correspond. For those who work at night, this becomes a chronic issue.

It’s this same lack of synchronicity that causes jet lag and “jet daze” when you fly across time zones.

Caffeine and alcohol
Many foods and beverages contain hidden caffeine. The half-life of caffeine is 7.5 hours, so beware of that late-afternoon latte, which can perk you up well beyond bedtime.

Wine with dinner may relax you, but it also interrupts the sleep cycle and causes midnight wakefulness.

Cigarettes
Cigarettes contain stimulants. Aside from long-term health damage and the possibility of burning down your house, smoking before bed or while in bed can cause a sleepless night.

Sleep apnea
Menopausal woman are as likely as men to suffer from this disorder. Symptoms include snoring, gasping, pauses in breathing and brief awakenings. All of these interrupt the stages of sleep. Sleep apnea is also correlated with high blood pressure, heart disease, stroke and driving accidents (plus daytime fatigue).

Limb movements
Periodic limb movement disorder is surprisingly common as we get older. Your legs inadvertently jerk and kick, making you wake during the night. This is similar to “restless legs syndrome,” where the legs also tingle and ache.

Night sweats
Nearly three quarters of women suffer from hot flashes during the first years of menopause. These may make you awaken soaked in sweat. You may not even be aware of these sweats, but they do interrupt sleep and lead to exhaustion the next day.

Pain, disease and medications
Anything that causes pain can interfere with sleep, as can chronic disease like arthritis, osteoporosis and heart disease. Another potential culprit is medication, including medications for asthma, attention-deficit disorder and depression.

Partner disturbances
If your bedmate snores, moves or gets up often during the night, this obviously can affect your sleep.That's the bad news. The good is that there are solutions to all of these problems.

If it is a question of time allotment, make it a priority to schedule enough sleep time. If you miss some sleep each night, it’s helpful to catch up on weekends, but this uneven pattern shouldn’t be your goal. It’s like bingeing on weekends and dieting during the week.

If you work nights, ask your doctor about Provigil, which promotes wakefulness. For other issues, also talk to your doctor. Changes in sleep habits and the right medications, if necessary, can make a big difference.

Dr. Reichman’s Bottom Line: Sleep is not a waste of time. It is vital for a functioning brain and your overall health.

Dr. Judith Reichman, the “Today” show's medical contributor on women's health, has practiced obstetrics and gynecology for more than 20 years. You willl find many answers to your questions in her latest book, "Slow Your Clock Down: The Complete Guide to a Healthy, Younger You," published by William Morrow, a division of HarperCollins.