As a neuroscientist who has spent her career researching sleep, Polly Moore never expected to have trouble getting a baby to nap.

But when her first child was born, Moore says, she felt as helpless as any other parent whose baby won't sleep. Although some parents urged her to put her daughter on a schedule, others told horror stories of failed nap schedules.

Then, when her daughter Maddie was 3 months old, Moore noticed a pattern. Maddie, now 10, seemed to become sleepy about 90 minutes after waking up each day. That reminded Moore of a 90-minute biological clock called the basic rest and activity cycle, which governs wakefulness throughout the day.

Although researchers have tracked the cycle in adults and children, Moore had never heard of anyone using it to schedule naps.

In "The 90-Minute Baby Sleep Program: Follow Your Child's Natural Sleep Rhythms for Better Nights and Naps" (Workman Publishing, $9.95), published in March, Moore has written an action plan for weary parents. It's based on babies' "natural lulls and sleepiness cycles."

Babies need their parents' help to sleep, she says, because the rest and activity cycle doesn't actually make them groggy. Instead, each cycle begins a new period of alertness. As that alertness fades, however, parents have a narrow window of opportunity - maybe five to 15 minutes - to put them to bed.

Parents should begin watching for signs of sleepiness - fussing, yawning, eye rubbing, ear pulling, staring into space - as their children's internal clocks are winding down, says Moore, director of sleep research at California Clinical Trials in San Diego, which runs scientific studies for businesses. Parents who fail to spot a child's sleep cues run the risk that the baby will go into another alertness cycle, too wired to sleep again until that cycle ends, Moore says. By that time, a baby is likely to be so sleep-deprived that he or she has even more trouble falling asleep.

After about four months, babies may stay awake longer, alert and uncomplaining for three hours or even 4 hours, Moore says. After a few days, parents may begin to recognize that a baby has his or her own internal schedule.

Pediatric sleep specialist Kim West says parents should pay more attention to their children's body language than to the clock. When babies are tired, she says, parents shouldn't wait to give them a nap. But the 90-minute clock could be helpful to parents who have trouble reading their baby's sleep cues, says West, author of "Good Night, Sleep Tight: The Sleep Lady's Gentle Guide to Helping Your Child Go to Sleep, Stay Asleep, and Wake Up Happy.

"Chelsey Anderson of Provo, Utah, says her 9-month-old daughter, Simone, hated to sleep. That left Anderson exhausted, with no time to clean the house or work. After reading Moore's book, Anderson, 25, says she was surprised to learn how much sleep babies really need - and to realize that a baby who seemed needy was really just overtired.

Reading Moore's book, she says in an e-mail, "was like reading about my own sweet, sleepy baby." Thanks to Moore's book, she says, Simone now goes to sleep at about 7:30 p.m., sleeps through the night and takes two to three naps a day.

THE 90-MINUTE PLAN

A new book says parents should follow their infant's natural.

90-minute sleep and alertness cycles to find the perfect time for naps. The N.A.P.S. Plan:

N— Note the time of your baby's last waking.

A— Add 90 minutes.

P— Play and pursue other activities with your baby.

S— Soothe your baby to sleep.

Source: "The 90-Minute Baby Sleep Program