Just as you slip into sublime slumber, you hear your baby's cries. So you do the nocturnal boogie down the hall, make it all better, watch your lovely child nod off, and, if you're lucky, get back to your own bed.

But not back to sleep, at least not right away. And there's a good chance you'll be awakened at least once more during the night -- or expect to be, which means your sleep is pretty much wrecked anyway.

So it goes when you're a mom. If there's one thing you can count on after the stork swoops in, it's kissing regular nights of uninterrupted, blissful sleep goodbye. Unfortunately, catching winks here and there between cries and whispers isn't the same thing. "Four two-hour sleeps is not equal to one eight-hour sleep," says Thomas Roth, Ph.D., director of the Henry Ford Hospital Sleep Center, in Detroit. "You'll end up with fragmented, inefficient sleep, which is functionally the same as sleep loss."

The day after a night like that, you may find it harder to remember things and to concentrate on work -- or your kids. Studies show that it doesn't take much sleep loss to affect how you feel. "Even one hour less sleep in a night can decrease your alertness by a third the next day," says Clete Kushida, M.D., director of the Stanford University Center for Human Sleep Research. Although you're not likely to get sick because your slumber is disturbed for a few days, a chronic lack of quality sleep isn't good for your health: If it drags on for months, it can make the immune system less efficient and impair hormonal regulation -- making you more susceptible to hormone-related diseases like diabetes. (Just what you wanted to hear after having a baby, right?) So to feel and function at your best -- as a mom, as a wife, as a human being -- you need not only enough sleep but good sleep too.

Imagine a perfect night's sleep. (You have a right to fantasize, don't you?) You'll go through several complete cycles of REM (rapid eye movement) sleep, alternating with non-REM sleep. Both kinds are important to your well-being. There are four stages of non-REM sleep, with distinct differences in brain-wave patterns. In the first two stages, your snoozing is rather light, as blood pressure and body temperature drop a bit. Soon you drift to stages three and four, for the deepest and possibly most restorative rest of the night.

Between these non-REM cycles you have REM sleep, when almost all dreams occur. Getting enough REM enhances your memory and mental sharpness the next day. A REM/non-REM sleep cycle takes about 90 minutes. Among healthy adults, there should be four to six complete cycles per night.

As the night goes on, the balance between non-REM and REM stages shifts. During the first few hours, you spend more time in non-REM sleep, particularly in the restful third and fourth stages. Later on, in the early-morning hours, you spend more time in REM, for increasingly longer periods. That's the ideal, anyway. For moms of young kids, these nice, predictable cycles fly out the window. Your sleep patterns become topsy-turvy. When you finally drag yourself back to bed after tending to your baby, "you'll probably drift off into stage one or two of non-REM sleep," Dr. Kushida says. "But if you're sleep-deprived or if you wake up frequently, you can get dumped into any stage of sleep."

It's easy to miss out on the deepest stages of non-REM and the longer stretches of dream-producing REM. After a while, you might not even remember what a good night's sleep feels like. "One of the most ominous things about being sleep-deprived is that you stop realizing you're sleep-deprived -- it's like being drunk," says Roth. "If you don't realize it, you can end up behind the wheel of a car, which could be dangerous. Sleepiness is a signal from your body to be careful of what you're doing."

But you don't have to take it lying down! Here, four common sleep-sabotaging scenarios moms are most likely to encounter, and how to triumph over each:

Stacey Colino, who writes often on health for Parenting, had her second child earlier this year.

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