Every era has the parenting obsession it deserves; ours happens to be sleep. Households divide between the nocturnally scheduled and the chaotic, and these days friendships can break up over the difference. "How did she sleep last night?" a mother acquaintance once asked, absently pushing a swing. But something about the mom's demeanor -- oxford shirt neatly tucked in, stackable snack packs -- made me feel a lot was at stake. If I told the truth, I might in her mind join the slothful drifters, in whose care the kids lie down when they may but are never actually "rested." So I cheerfully answered "Fine" and kept swinging.

In the midst of the sleep wars comes a study that gives a boost to the indulgers of chaos. A survey reported last month that the percentage of infants who share a bed with an adult has doubled in the past seven years, to 12.5 percent of Americans. Until now the practice of co-sleeping has been seen as fringy and faintly embarrassing, the sad lot of tenement dwellers, or a crusade of La Leche League types and evolutionary biologists who think mothers should model themselves on Jane Goodall. Now we know some of our friends were just failing to come clean.

The survey, done by the National Institute of Child Health and Human Development, validates at least the class stereotype. Poor mothers under 18 are much more likely to sleep with their infants, and to do it in awkward places such as couches and cots. But the study showed a jump in the practice overall, with 50 percent of infants studied having spent some hours in an adult bed during the previous two weeks.

I am not a regular co-sleeper, but I can understand the appeal. For the poor it must be about a practical division of available space: Better to have the infant keep you up than keep your other children up. For everyone else it seems just another symptom of the juggling life. A day spent racing from work to stovetop to bath leaves little room for intimacy, so busy parents are tempted to steal a long stretch at night. Plus there are few greater pleasures than seeing your child's face at the instant of waking up, sunk and tousled and then suddenly smiling.

In parenting literature, co-sleeping is usually mentioned as an extreme hazard, up there with plastic bags and toys with small parts. But the warnings that parents who co-sleep risk smothering their infants never mention that in the great majority of those cases the parent, nearly always the father, was drunk or high.

Ultimately what's most satisfying about this latest co-sleeping study is knowing that parents are bucking the dogma of experts. Ann Hulbert just finished a book, "Raising America," in which she challenges the notion that generations of parents have slavishly obeyed the reigning experts. If so, this study bears her out. These days, whether they are considered right or left, experts present their positions as the certainties of "new neurological science." They take the no doubt important but pretty obvious recent research showing that children's sleep needs are different from adults'. Then they lift that information to the level of social manifesto.

The favorite among my friends is the best-selling "Healthy Sleep Habits, Happy Child," by Dr. Marc Weissbluth. The book is full of charts and graphs that add up to a "step by step program for a good night's sleep," down to the exact number of minutes for an appropriate bedtime routine. An hour more of sleep breeds a child with a "longer attention span," a poetic angel who learns "simply from looking at the clouds and trees, touching, feeling, smelling, hearing." An hour lost means children who are "fitful and socially demanding, less able to entertain or amuse themselves." Personality and parenting play minor roles in the outcome. What matters is REM sleep.

And we're not just looking at the early years. A missed nap here, a late night there, and you can kiss Harvard goodbye. "Small but constant deficits over time tend to have escalating and perhaps long-term effects on brain function," he writes in one of his many dire boldface quotes. Sprinkled in are testimonials from grateful parents who finally learned they must stop being selfish and put their child to bed at 6:30: "Dr. Weissbluth's advice was right on the money!"

I know Weissbluth cultists who live in small apartments and who lock themselves in their kitchens at night lest their footsteps interrupt a stretch of deep sleep; others who have rid themselves of a beloved family dog that might bark at the wrong time; still others who in three years have not left the house after bedtime. Next to that, a little chaos seems relaxing.

The writer is a member of the editorial page staff.