Little kids give up naps at different ages. A new theory links that transition to brain development.

kamel ADJENEF/Photographer

Parents watch for certain milestones as their young children grow: their first words, their first steps, and when they stop napping.

But while walking and talking start at predictable ages, when a child stops taking a nap varies widely, anywhere from age 2 to age 7, said Rebecca M.C. Spencer, professor of psychological and brain sciences at the University of Massachusetts Amherst.

She and her colleagues set out to understand why. In an article published Monday that ties together her and other scientists’ earlier research, they posit that brain development, not age, as many parents might expect, is the reason for that variation.

Spencer’s research underscores the importance of providing all young children with the opportunity to sleep during the day, she said.

Children’s brains can hold only so much information; they need naps to organize and retain all the stimuli and sensations that bombard them, she explained.

“When they don’t nap, they actually show memory loss,” said Spencer, who cowrote the article in the Proceedings of the National Academy of Sciences, with Tracy Riggins, a University of Maryland child psychologist. “It’s detrimental for them to stay awake during nap time.”

But children whose brains have matured to the point they don’t need to nap “can stay awake without this catastrophic effect on memory,” Spencer said.

Spencer and Riggins have spent years studying the relationship between children’s sleep habits, their performance on memory tests, and the rates at which different areas of their brain mature. A study they published in the journal Nature in 2020 found that children who had transitioned out of napping had a more developed hippocampus, the area of the brain involved with memory and learning, than children who were still sleeping during the day.

The hippocampus undergoes tremendous growth during early childhood, and that growth occurs at a different rate in each child. It also occurs during the same period as the transition away from daily napping.

“We know there are key developmental patterns happening at this age . . . in ways we think are relevant to how children sleep,” Spencer said.

School leaders don’t always recognize the importance of naps, Spencer said. California’s new universal pre-K program does not require naptime. In New York City, naps are suggested, but not required.

In Massachusetts, preschool programs are required to give children a 45-minute “rest opportunity.” Spencer finds this both vague — what is a rest opportunity? will the lights be on? — and inadequate. Most preschoolers need 70 to 90 minutes of daytime shut-eye, she said.

When children are deprived of needed naps, “you’re slowing down the cognitive development. Day to day they’re not remembering enough,” she said.

Sleeping more at night won’t make up for it. “The sleep has to occur close to learning,” Spencer said.

The transition out of naps isn’t like flipping a switch, she cautions. Kids may skip naps for a while, and then resume them.

Spencer hopes her theory, drawn from previous research of hers and others, will prompt more studies of children’s sleep. If a way to assess hippocampus functioning could be developed, parents and teachers might be able to identify which children still need naps and which don’t. Meanwhile, she said, “It’s best to encourage all kids to have a quiet time and settle down to see if they can nap.”

How can a parent today tell whether their child no longer needs to nap? Currently parents tell by observing that naps become less frequent, shorter, and that a child’s behavior becomes less challenging when they miss a nap, she said. “But telling these cues apart from a child who is just being defiant about taking a nap . . . can be hard,” she added.

Rebecca Gómez, a professor of cognition and neural systems and director of the child cognition lab at the University of Arizona, notes that even children who have stopped napping sometimes need to snooze when faced with challenging concepts. “When the information is more complex, children can’t form a strong memory during the learning experience. Those children may still need a nap,” she said.

Spencer’s essay is raising “all sorts of intriguing and important questions, and really puts this issue of napping on the map for parents,” Gómez said. “This theory will be very important in driving research.”

But she suspects that more than just the hippocampus is involved.

“It really needs to be broadened out to all the developmental changes that are happening,” Gómez said. “I don’t think it’s just the changes in the hippocampus that are driving this.”

“Sleep affects all systems in the body and the brain” — not just memory but social and motor development as well, Gómez said.

Mary A. Carskadon, director of the Chronobiology and Sleep Research Laboratory at Bradley Hospital in East Providence, R.I., called the essay “well-put and a good theory” that draws on a wealth of previous research. But like Gómez, she thinks “there are other things probably going on,” such as a genetic process.

“What accounts for individual differences in this process, and how much of it is neurobiology and how much of it is culture and environment?,” she said. “The best kinds of theories are those that are put out like this and can be tested.” The next step would be to follow a group of children over time and measure their napping and memory.

“One of the most important takeaways is that we need to support napping in children,” Carskadon said.

Spencer agrees fervently with the need for more research and laments that so few are examining the connection between sleep and cognition in children. While many researchers are studying sleep disorders, “the function of healthy sleep, we don’t know a lot about,” she said.