You Might Be Asleep, But Your Brain Is Not!

Your parents might not let you stay up late at night—but neither will your body! When we sleep, our bodies heal, restoring our energy and “packaging” our memories from the day. Memory lets us travel back in time in our minds. We can remember events, activities, and information from earlier in our lives. Sleep not only protects our memories, but it also helps save our memories for later, which is known as memory consolidation. Without proper storage, our memories are fragile and can be easily forgotten. In adults, memories can be stored with a good night’s sleep. However, in young children, naps act like “pit stops”, packaging memories more often to help children remember the day’s events. Infants and toddlers take multiple naps a day because of how much they learn and how much their brains grow. As children start going to elementary school and their brains “grow up”, they take naps less often. You might remember this from your own life: when you were in preschool, you might have had nap time. But, when you got to elementary school, you probably did not have nap time anymore. These are changes teachers make, but how do they know what best helps children learn and remember?

How Does Napping Change As We Get Older?

Newborns sleep for 20 h each day, but not all at once. Instead, they sleep many times during the day. By 4–6 months, most babies take three naps during the day and then sleep for a longer time at night. At 9 months, most babies move to two naps and nighttime sleep. By the time children are 1–2 years old, they usually have just one nap plus nighttime sleep. Sometime between 2 and 7 years, naps go away, and children sleep like adults—just overnight! This transition out of nap times is different for every child.

Why It Is Hard To Sleep During The Day

Why do we sleep at night and stay awake during the day? It all boils down to an internal machine in our bodies known as the circadian clock. The circadian clock is like a regular clock, except it goes around every 24 h instead of 12 h. During certain times of the day, our bodies release special chemicals that either help us sleep or stay awake. The circadian clock is sensitive to temperature, sound, and most importantly, light! At night, when the sun is down and there is no light, our bodies release a chemical called melatonin, which makes us sleepy. In the morning, when the sun is up and there is plenty of light, our bodies release other chemicals that help us stay awake.

In addition to an internal clock, our bodies also have an internal homeostat. A homeostat is like a thermostat that keeps our homes at a steady temperature. Our homeostats make sure that we have a stable environment in our bodies. For example, if the temperature within our bodies increases, the homeostat triggers sweating to bring it down. If the temperature within our bodies decreases too much, the homeostat brings it up by shivering. The same can happen for sugar levels, blood pressure, and sleep need.

How Do The Circadian Clock And Homeostat Work Together?

Both the circadian clock and homeostat keep our bodies healthy and active (Figure 1). From going to school, playing outside, or doing any daily activity, the circadian clock and homeostat work together to ensure that we can maintain an internal balance and perform tasks smoothly and on time.

If you always followed your circadian clock, you would fall asleep at the same time every day, regardless of when you woke up. However, this is not the case. For example, think of a time you stayed up late to do homework or watch movies with friends. If you only had a circadian clock, you would wake up the next day at the same time you always do, and go to bed at your usual time the next night. But you probably woke up later and wanted to sleep earlier the following night because you were so sleepy! That is your sleep homeostat at work: the longer it has been since you slept, the more homeostatic sleep pressure you have. This pressure goes down only when you sleep again. Slow wave activity (SWA) is a type of brain activity during sleep that is associated with sleep pressure. It is highest at the beginning of sleep but decreases as you sleep, and it is also higher when you are sleep deprived (such as when you stay up late). SWA is also related to learning, so more SWA accumulates in parts of the brain that have been busy during wake.

In young children, sleep pressure builds up faster than it does in older children. We think sleep pressure is related to why young children need to nap more at younger ages and why they nap less as they get older.

Why Does Sleep Pressure Change With Age?

To understand why sleep pressure changes with age, we came up with a hypothesis [1]. Storing memories temporarily fills up the hippocampus, an area of the brain that is really good at holding memories in the short-term. The hippocampus fills up with memories similar to a bucket filling up with water (Figure 2). In younger children, their “bucket” is small and fills up quickly with memories and would also have high SWA (since SWA is associated with learning). When children nap, they can empty their memory buckets and reduce their sleep pressure (that is, SWA). As children get older and their brains change, the hippocampus “bucket” grows larger and can hold more memories, and accumulate less SWA, without sleeping as much. In other words, with bigger buckets, older children do not need to nap as much, because they have more space to fill. Eventually, when the bucket gets big enough, we can stop napping all together and wait to empty it until we go to sleep at night!

What Evidence Supports This Idea?

Three main kinds of research evidence all support the idea that naps are related to storing memories.

First, children who have grown out of their naps have better performance on memory tests than children who naturally still nap every day, even if these children are the same age [2]. Non-nappers remember more numbers and words, which shows that their ability to hold memories is stronger.

Second, children who naturally nap every day forget things more quickly when they skip their naps compared to children who have stopped napping, even if they are the same age [3]. Remember, napping helps children “dump out” memories and reduce the sleep pressure (or SWA), so their brains can collect new memories as they move through their day. Picture this: you have two children that are the same age, one who still naps every day and one who does not. Both of them are asked to stay awake, even if the napper really wants to take a nap. What happens? The napper will actually forget more of their memories than the non-napper! This may be because the napper cannot hold all their memories in their bucket for so long—they need the nap to help them keep their memories. However, children who have stopped napping can hold all these memories for a longer time. So, they do not need a nap to protect their memories.

Third, children who naturally nap every day have different-sized hippocampuses compared to children who have stopped napping, even if they are the same age [4]! Remember, as our brains develop, the size of our “bucket” changes. This supports the idea that nappers and non-nappers store the day’s information in different ways.

Can The Environment, Culture, Or Parents Make A Difference?

Parents often work with the circadian clock and homeostat to get their children to nap. For example, parents often have a consistent nap schedule and darken the room when they want their children to nap. But some parents do not want their children to nap, either for cultural reasons or because the child will be starting school soon. So, they keep the room well lit, and do not encourage their children to nap. But what does preventing nap time mean for children’s memories and brain development? What if the child’s memory depends on those naps to develop? That is the next question we want to address with our research, and one reason why it is important for parents and educators to know about what sleep does and why it matters in children’s early development!

Glossary

Consolidation: ↑ The process of packaging our memories and “moving” them to different parts of the brain to be stored long term.

Circadian Clock: ↑ The body’s internal clock, which keeps you in time with the 24-h day/night cycles.

Melatonin: ↑ A chemical produced by the body that makes one feel sleepy and prepares them for sleep.

Homeostat: ↑ Parts of the body that maintain a state of balance within the body. This can apply to temperature, blood pressure, and sleep.

Homeostatic Sleep Pressure: ↑ A sleepy feeling that builds up the longer one stays awake.

Slow Wave Activity (SWA): ↑ Activity in the brain that is thought to reflect the amount of “pressure” there is to sleep.

Hippocampus: ↑ A part of the brain responsible for memory and learning.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.