This post is part of our series on Infant Sleep and its Impacts on Development, published in collaboration with the journal Infant Behavior and Development. The featured research appeared in a special issue on how infant sleep affects cognitive, social, and physical development and how parents and practitioners can help promote healthy sleep and development in infancy.
For adults, naps offer a period of solace, something we typically allow ourselves only on weekends or vacations. We long for these moments to replenish lost sleep due to our busy lives and to reset our minds after being overloaded with to-do lists.
Science supports the benefits of naps. In adults, naps benefit attention, emotion, and cognitive performance. Other researchers and I have shown that these benefits extend to naps in early childhood. For instance, the benefits of naps for memory have been observed in infants as young as three months and extend through three to five years, when children typically transition away from naps.
It is tempting to assume that naps help memory just by blocking out other stimuli that interfere with memories. Later in the day, we may forget the name of a person we met in the morning simply because we encountered so many other names and faces during the subsequent time awake. Distributed sleeping may protect us from such interference. However, the benefits of sleep extend beyond just protecting memories from interference with ongoing learning.
Naps have been consistently shown to support memory consolidation, even in early infancy.
While we sleep, memories are strengthened through a process called consolidation. When we learn something, the memories are initially stored in the hippocampus in the brain. The hippocampus is small and not a very “smart” storage area – all memories get put into a single bucket regardless of their content (e.g., memories of family, a book you read, and your work are all in one place).
When we sleep, these memories are replayed. Memory replay is akin to rewatching (or replaying) the “movie” of your day. Just like repeatedly watching a scene from your favorite movie to learn all the words, the hippocampus replays memories while we sleep.
Replaying the memory makes a copy of the memory that is stored in the cortex. The cortex is a much smarter organizing system – more like a filing cabinet where similar memories can be stored together. This makes it easier and quicker to recall memories from the cortex later.
Parents and caregivers may wonder how often their child should nap during the day and whether having more than one nap makes a difference in their child’s development. Infant sleep is initially distributed across multiple naps (polyphasic sleep) but by nine months, most infants regularly have just two naps a day (triphasic sleep).
The transition to one nap a day (biphasic sleep) typically occurs between 12 and 18 months. The transition to adult-like monophasic sleep (no naps) occurs between three and five years for most children.
Given the presence of multiple naps within a day in infancy, my colleagues and I were interested in whether different naps aid memory in similar ways. On the one hand, naps have been consistently shown to support memory consolidation, even in early infancy. In this case, naps at any time of day may have significant memory benefits.
On the other hand, sleep physiology has not been compared across distributed naps. The morning nap, which infants “grow out of” first, may not be enriched with the distinct brain waves that support memory. In other words, the morning nap may not have significant benefits for memory.
In our study, we assessed memory in nine-month-olds. We used a deferred imitation task, which is commonly used in developmental psychology to assess memory. This task is similar to how parents engage their infants with a new toy.
An experimenter shows the infant an unfamiliar toy and demonstrates a certain set of actions. Then the infant is given another toy, the target toy, and has the opportunity to imitate those actions. If the infant imitates the actions, this is evidence of their memory for initial demonstration. We also used control procedures to make sure the actions we were looking for were not simply the natural intuition of the infant when engaging with the toy.
Even though they napped in the afternoon, when the infants stayed awake during the morning nap, they forgot more after the afternoon nap than they did when they had had a morning nap.
In our study, 15 infants were presented with four target toys and we measured their immediate recall of the demonstrated actions to find out if they imitated the target actions with the toy. Next, the infants napped during their morning naptime.
After their naps, they were given the toys again to see if they demonstrated memory of the experimenter’s earlier actions by imitating them. To compare the infants’ actions with and without naps, we also carried out the study the week before or the week after the nap study with infants being kept awake during their morning naps.
Infants’ memory was protected when they took a morning nap: They tended to remember just as many items after their morning nap as they did before the nap. However, when infants stayed awake during their morning nap, they forgot some of the items.
Next, we considered whether staying awake during the morning nap affected infants’ memory consolidation during the afternoon nap. The infants were presented with a new set of toys and then took their regular afternoon nap. Even though they napped in the afternoon, when the infants stayed awake during the morning nap, they forgot more after the afternoon nap than they did when they had had a morning nap. That is, taking the afternoon nap did not compensate for a missed morning nap.
Our study points to the importance of naps for learning in infants. Memories are protected by naps at this age when learning is vast – from the faces of caregivers to the intricacies of language. Moreover, later sleep does not compensate for a missed nap. Instead, the effects of a missed nap can be compounded by damaging the function of later sleep.
In our ongoing work, we are manipulating the presence of the afternoon nap to directly compare memory loss when babies are kept awake during the afternoon and during the morning nap. We are also studying these infants longitudinally to understand how the function of naps changes when the morning nap becomes less essential. By recording brain activity during distributed sleep, we also aim to better understand the relation between memory and brain development.
Promoting sleep health in infancy and childhood is crucial for cognitive development. This is particularly important for families with low socioeconomic status who may lack knowledge about babies’ needs for sleep and resources to provide opportunities for sleep in the middle of the day. We need to continue our work to understand the function and timing of distributed naps so we can provide guidelines to caregivers and practitioners.
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