The sleeping brains of infants and toddlers are focused on learning and memory support until about 2.4 years of age, when brain activity shifts to spend more time on maintenance and repair, according to a new analysis published in the September 18 issue of Science Advances.
Most surprising to the study authors was how suddenly the change appears to occur. Computational biologist Van Savage, a professor at UCLA and senior author of the paper, described it as a phase transition akin to water turning to ice. Sleep patterns change at other stages in life such as during puberty, but not as markedly as they appear to between two and three years old. The findings improve scientists' understanding of how sleep affects childhood development, the study authors say, and offer an important contribution to debates about the function of sleep.
Though humans spend about a third of their lives asleep, the primary function of sleep and why it's so pervasive throughout the animal kingdom has been unclear. Though it may seem intuitively that sleep is for rest, the amount of energy saved during sleep is only about what one would get from eating one hot dog bun.
"That's definitely different than if you were sprinting, but it's not that much different than if you were sitting in a chair all night and reading," said Savage. Being unconscious and vulnerable to predation or other threats for eight hours each night, he said, is not worth the energy savings.
Brains engage in two activities that may require unconsciousness, however: maintenance and repair of the stresses of normal function, and the reorganization of its cells and their connections to support learning and memory.
Babies in particular benefit from neural reorganization, which appears to happen in rapid eye movement (REM) sleep, when the brain is most active and the eyes flutter beneath their lids. Brains encode lessons and memories by creating unique networks of brain cells, connecting them at junctions called synapses.
Babies' neurons are extensively connected, making them very open to learning, but the lack of focus or reinforcement of particular neural pathways means babies lack mastery in things like language, motor skills and vision. Humans may have evolved to learn those things after birth since they are born in variable environments. If babies hear adults speaking English, their sleeping brains later prune away the synapses that would help them recognize sounds in Chinese, for example, and reinforce the ones encoding English sounds. This is also how babies learn about motor skills or cause-and-effect.
"And laughter!" said sleep neuroscientist and paper co-author Gina Poe, also a professor at UCLA. "The very first smiles that you see in an infant often occur in REM sleep days before they occur in wakefulness."
While it may seem appealing to keep the brain open later in life to more easily learn new skills or foreign languages, Poe said the extra synapses are costly for the body to maintain, and that openness would keep people from making good judgments or choices. Adult brains do continue to reorganize throughout life, but not as readily as baby or toddler brains.
Whether sleep is primarily for reorganization or for maintenance is a topic of ongoing debate in sleep research. Sleep has been mysterious to scientists for two main reasons, said Savage.
The first is due to the nature of sleep. "It's not easy for everyone to sleep while they're inside an MRI machine," he said. Brain activity, gene expression and cell signaling are all difficult to measure without disrupting sleep, and sleepers can't provide good reports of their sleep experience.
Secondly, Savage said, sleep research has been understood primarily in terms of verbal, qualitative models instead of rigorous mathematical modeling and analysis. While it is clear that babies sleep more than adults and spend more time in REM, scientists have not clearly established the mathematical relationship between age and sleep activity, making it harder to investigate thoroughly.
To study precisely how sleep changes throughout early childhood, Savage and his team developed mathematical models relating measures like metabolic rate, sleep time, REM time, brain mass and age to maintenance and reorganization activity. They then drew on 400 data points from previous sleep studies on children from 0-15 years old.
They found that neural activity associated with neural reorganization and REM sleep is most prevalent before about 2.4 years of age, when babies spend about 50% of sleep in REM. Beyond this point, reorganization stabilizes, REM time reduces to about 25%, and activity associated with brain repair becomes dominant.
As scientists confirm and better understand this shift, said Poe, it may become an indicator to track and assess the development of young children.
"If there's a fairly sharp transition around two and a half and you don't see that in a particular child, we can ask, 'What does that mean?'" Poe said. "That they're especially smart and they've got lot more neural reorganization that needs to happen? Or is it indicative of some sort of developmental delay?"
Poe and Savage say that this transition should be studied more directly both in humans and in animals, perhaps using new technologies like unobtrusive wearable devices that can monitor sleep.
In the meantime, Savage hopes this research will help people understand the critical processes that happen during sleep.
"Sleep serves a vital need so you should value it like eating or breathing and take care to get it," he said. "But I don't want to cause people extra anxiety or stress if their kid's not sleeping. Because my son, to be honest, was a terrible sleeper."