Quite a bit of research has begun to explore influences of sleep on cognitive processes. In adults, sleep has a huge influence on memory. Sleep speeds learning of new skills. It also helps to separate the information being learned from the situation in which it was learned, which can make it easier to use that knowledge in other circumstances.
Young children spend a tremendous amount of time asleep, and so research is also beginning to explore the influence of sleep on things children learn. An interesting study in the March/April, 2014 issue of Child Development by Denise Werchan and Rebecca Gomez examined how sleep influences toddlers’ ability to learn new words.
When a child learns a new noun, for example, it is important for the child to be able to apply that word to the object (or objects) for which they have seen it used, but also to apply that word to other objects that come from the same category. For example, a child may sit in the family minivan and hear it called a car. She may see a neighbor’s sedan and hear that called a car as well. She might also be given a four-wheeled toy and hear that is a car, too. To be a successful language-user, this child ultimately has to be able to recognize which other objects should be called a car and which ones should not.
This process requires generalization. That is, the child has to go from the limited number of instances of the category they have seen to figure out which other objects share the same label. This process requires some amount of forgetting. After all, the child will observe many characteristics of these objects like their shape, size, color, and parts. Some of these characteristics (like shape, and some parts) matter a lot in deciding whether to call something a car, and others (like color) matter less. So, it is helpful for the child to forget some of what they have seen in order to begin to generalize the new word to other objects.
Werchan and Gomez suggested that sleep might actually interfere with toddlers’ ability to learn new words. These researchers argued that sleep helps to solidify memories, and so if children associate too much information with a label, they might not learn to generalize it to new objects.
To test this possibility, 30-month-old toddlers were taught labels for three types of novel objects (which were constructed by the researchers). The labels were words like dax or tiv that are not used as words in English. During the training, children saw three examples of each object. They were also exposed to several other novel objects that were not labeled, that would be used as distractors later.
One group of children was tested about an hour before their normal nap time. They napped, and then came to a psychology lab to be tested four hours after the training. A second group was tested far from their normal nap time. They were also tested in the lab four hours after training, but they had not napped. A third group was trained and then tested immediately.
During the test, children saw four objects: a new example from one of the categories they learned, an object they saw during training that had not been named, an unfamiliar object, and a familiar object (like a toy duck). They were told the label and were asked to point to the object. For example, if they saw the object that had been called a dax during training, they would be asked “Which one’s a dax? Can you point to the one that’s a dax?”
The children who were tested immediately and those who napped got about 40% of the test questions correct. The children who did not nap got over 80% of the test questions correct.
This study suggests that when toddlers are learning words, their ability to generalize those words to new objects requires them to forget some of what they saw. More of this forgetting happens when children remain awake than when they sleep. So, this kind of word learning is enhanced when children stay awake after learning the words.
As the researchers point out, this finding differs from what is usually observed with adults. Adults often generalize their learning better after sleep. The difference is that adults are better than toddlers at focusing on the most important information when learning something new. So, for adults the most important part of generalizing is separating the content of what was learned from the situation in which it was learned. Sleep helps with that separation. Toddlers need to forget some of the content of what they learned in order to generalize effectively, and so for them sleep helps them.