Few features of child-rearing occupy as much parental brain space as sleep, and with it the timeless question: Is my child getting enough? Despite the craving among many parents for more sleep in their offspring (and, by extension, themselves), the purpose that sleep serves in young kids remains something of a mystery—especially when it comes to daytime naps. Do they help children retain information, as overnight sleep has been found to do in adults? A study published today in the Proceedings of the National Academy of Sciences provides the first evidence that daytime sleep is critical for effective learning in young children.

Psychologist Rebecca Spencer of the University of Massachusetts (UMass), Amherst, had more than a passing interest in the subject: Her daughters were 3 and 5 when she began chasing answers to these questions. She also wondered about growing enthusiasm for universal public preschool, where teachers don’t necessarily place much emphasis on naps. “There is a lot of science” about the best curriculum for preschool classrooms, “but nothing to protect the nap,” Spencer says. Still, data to support a nap’s usefulness were scarce: Studies in adults have found that sleep helps consolidate memories and learning, but whether the same is true of brief naps in the preschool set was unknown.

So Spencer approached the first preschool she could think of that might help her find out: her daughters’. She later added other local preschools to her sample, for a total of 40 children ranging from nearly 3 to less than 6 years old. The goal of Spencer, her graduate student Laura Kurdziel, and undergraduate Kasey Duclos of Commonwealth Honors College at UMass, was to compare each child against him or herself: How well did a child learn when she napped, and what happened when she didn’t?

To test this, the trio first taught the children a variant of the popular game Memory or Concentration. They were shown a series of cards with pictures on them, such as a cat or umbrella. The cards were then flipped over, hiding the pictures. Each child was offered another picture card and asked to recall where in the matrix its match lay.

Then, about 2 hours later, it was naptime—or nap-free time, depending. Kurdziel and Duclos developed various “nap promotion” techniques, resting a hand on a child’s back, rubbing their feet (this was surprisingly effective), or simply sitting next to them. “If they know that someone’s got their eye on them then they can’t wiggle around as much,” Spencer says. The average nap was about an hour and 15 minutes.

Soon after the children woke up, the memory game was repeated. On a different day, they learned the game in the morning, were deprived of a nap, and then tested again. All participants repeated the memory game the next morning, too.

A nap made a notable difference in how the preschoolers performed, especially among those who were used to getting one. After sleeping, a child scored just as well on the memory game as she had before her nap. If she didn’t sleep, her score dropped 10% on average—so a child who correctly recalled where 75% of the cards sat would guess 65% if napless. More surprising to Spencer was that this pattern endured into the following day—scores held steady in the nappers, but had the same deficit in non-nappers. The researchers hadn’t kept track of overnight sleep, but it was apparent that even if a child caught up a bit at night, that didn’t provide a memory boost. “This is really saying, they need that sleep close to learning” for it to take root, Spencer says. The researchers also rated “child sleepiness” after naps and no naps, and concluded that the nappers were actually drowsier when the game was repeated—suggesting that their performance wasn’t improved just because they felt refreshed from a nap.

The children who normally napped at least five times a week suffered the most memory loss without a nap, forgetting about 15% of what they’d learned. Those who rarely napped didn’t benefit from the bonus nap, suggesting that they’d matured out of needing one and could get by with a bit less sleep and a longer awake stretch. Kids likely abandon naps at different ages because “their brains are maturing at different rates,” says Sara Mednick, a psychologist at the University of California, Riverside, who wasn’t involved in the study. That the learning difference endured into the next day, Mednick says, confirms “that [for] those kids who are still napping, their brains are not ready to be awake for 12 hours.”

To probe the nature of nap-sleep, Spencer and her students had 14 of the children, including one of Spencer’s daughters, take a nap in their sleep laboratory while recording brain wave patterns. They found what are called “sleep spindles,” short bursts of brain activity that have been associated with memory processing. “We’re at the infancy in really understanding how memories are consolidated, encoded” during sleep, says Mednick, who published a study earlier this year showing that more sleep spindles, thanks to a sleeping pill in adults, correlated with improved memory after sleep. She and Spencer both believe that the new work underscores how critical naps might be to early childhood learning—and that preschools need to make it easy for children to take them. In fact, some of the youngsters in the study were so miserable when forced to stay awake that the researchers took pity on them, let them conk out, and excluded them from the analysis. “If they need the nap,” Spencer says, “they should get the nap.”