A newborn’s job is to learn and adapt to everything in their environment, says William Fifer, a developmental neuroscientist at Columbia University. Nevertheless, newborns spend around 70 percent of their time sleeping. So Fifer and developmental psychologist Amanda Tarullo, now at Boston University, decided to see if they could see that learning happens while babies sleep.
The researchers found that infants as young as 1 or 2 days old can learn that a sound predicts a gentle puff of air. Eventually, after just hearing the sound, the infants blink, much like how Russian physiologist Ivan Pavlov’s famous dogs drooled in response to certain sounds, initially followed by food.
In adults, the importance of sleep for learning is well documented. But much less is known about how sleep and learning interact in newborns, or how that relationship changes as infants grow into preschoolers.
Naps are particularly puzzling. Studies suggest they are crucial for early learning, but most children naturally stop napping between the ages of 3 and 5. No one knows why, but researchers’ emerging understanding of napping and learning could help parents, preschoolers, and policymakers make better decisions about children’s health and learning.
Sleep patterns change dramatically as children develop. Newborns sleep about 16 to 18 hours a day. Initially they sleep randomly throughout the day, but after about six months their internal clocks sync up to the day-night cycle. By around 12 months, infants are mostly napping at night, with a few naps during the day. By the age of two, most children only have one nap a day.
Research suggests that naps are important in infant learning. “Sleep is crucial for early word learning,” says Manuela Friedrich, a neuroscientist at Humboldt University in Berlin.
In a 2015 study, Friedrich’s team presented 90 infants aged 9 to 16 months with pictures of unfamiliar objects — things that looked like dumbbells or tinker toys, for example. When looking at each picture, the children heard the name of the object – a made-up word like “bofel” or “zuser”. An hour or two later, the researchers showed the infants the images again, along with either the object’s “name” or another made-up word.
The researchers examined electroencephalogram (EEG) recordings for evidence that the infants made the connection. Previous research had identified certain features of an EEG trace – like a voltage jump – that appear when people hear something unexpected. In Friedrich’s study, the EEG recordings suggested that children who napped before the tests recalled the word-object pairs they had previously seen. Non-Nappers didn’t.
Not only that, the dozing infants appeared to group the learned objects into categories: When they saw new objects that resembled those they had previously seen, their EEG activity suggested they were expecting a previously learned word.
In the same study, Friedrich and her colleagues also found that babies’ ability to categorize objects correlated with an EEG feature during naps called sleep spindles: rapid bursts of electrical activity. Higher amplitude sleep spindles—those with larger swings in tension—only occurred in children who learned to generalize the words.
These sleep spindles often co-occur with slow wave sleep, a specific frequency of slowly oscillating EEG activity. In adults, slow waves occur during deep sleep and have been linked to memory consolidation, the process by which short-term memories are converted into more permanent memories. But children experience more slow waves than adults during naps.
Naps can help teens solidify memories as they learn vast amounts of important information. “Science seems to indicate that napping plays a unique role in early cognitive development,” says Simona Ghetti, a developmental psychologist at the University of California, Davis and a co-author of a 2020 article in the Annual Review of Developmental Psychology on memory in the developing brain.
Napping also seems to help babies understand sentence structure.
When cognitive psychologist Rebecca Gómez of the University of Arizona in Tucson and colleagues exposed 48 15-month-olds to a made-up language containing word sequences like “vot wadim jic” and “vot kicey jic,” sleeping children seemed better able to recognize patterns grasp, such as understanding that the first word of a sentence always determines the third word. The researchers see this as a step towards understanding the grammar.
For older children, naps can help with more than just language. In a 2020 study, Rebecca Spencer, a cognitive neuroscientist at the University of Massachusetts Amherst, and her graduate student Sanna Lokhandwala mimicked a shared preschool activity: the reading lesson. They created several books that told a short story – for example about a day at the zoo. In Spencer’s sleep lab, a researcher read these books to 3- to 6-year-olds while pointing to the pictures.
Immediately after reading a story, the researchers asked the children to arrange a series of pictures from the books. Then some children would nap for up to two hours while others stayed awake. The researchers found that naps more accurately recalled the order of events in a second memory test. These children also performed better on the same test the next day.
When children who nap regularly miss their nap, they tend to do poorly on memory tests, researchers have found. “Naps are great — they do all these important things for kids at a really critical time,” says Spencer.
But kids eventually give up their regular naps. “Why stop napping when it’s so important?” she adds.
Since children stop napping over a fairly broad age range of 3 to 6 years, she suspects the answer may have to do with brain development: maybe at some point the brain changes in ways that make napping fun makes learning less important.
Some evidence for this comes from Spencer’s collaboration with Tracy Riggins, a developmental neuroscientist at the University of Maryland at College Park. Using magnetic resonance imaging (MRI), they scan children’s brains and focus on the hippocampus, a part of the brain important for creating new memories.
Spencer and Riggins hypothesize that hippocampal development drives the transition away from regular naps. Research in laboratory animals and adult humans suggests that the hippocampus acts as a short-term storehouse for new information learned during waking hours. During sleep, these memories are transferred to the cerebral cortex for longer storage.
Spencer compares the hippocampus to a bucket. Early in development, when the memory bucket is small, it needs to be emptied more frequently, during sleep. Small children do this by taking a nap. When young children, who habitually nap, are deprived of a nap, they forget a lot of information, Spencer says. But as the hippocampus develops, so does its capacity.
“As I mature and have a larger hippocampus, I can hold more without having to empty my bucket,” she says. Spencer and Riggins believe this may explain why the need for naps decreases as children get older.
Decisions for parents, kindergartens
Researchers hope findings like these can help improve education and sleep in young children, and help policymakers and parents make better nap decisions.
In the United States, politicians and school leaders have often argued that preschools should focus on teaching, not sleeping. And in some parts of the country, educational standards have changed to reflect this idea. For example, in Massachusetts, where Spencer works, teachers tell her that the amount of time allotted for naps in preschool has decreased over the past 10 to 15 years. The standards for sleep-promoting classrooms have also relaxed, for example by allowing teachers to darken rooms during lunch breaks. In some schools, parents can send younger kids to swimming lessons or Zumba fitness classes instead of napping, Spencer says. She is concerned about the impact this is having on learning.
For very young children who habitually nap, missing a nap harms their cognitive performance — they lose information they learned in the morning without an afternoon nap, Spencer says. She wishes parents were more aware of the benefits of napping.
“If someone makes a policy decision or you let the parents choose between the dorm room and the no-sleep classroom, I think that should be an informed decision,” she says.
Carolyn Wilke is a Chicago-based freelance science writer.
This article originally appeared in Interesting Magazinean independent journalistic endeavor by Annual Reviews.
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