Why do we sleep? The answer may depend on the person鈥檚 age, according to research that suggests the main role of sleep changes at the age of around two-and-a-half.
Newborn babies spend a lot of time sleeping, and this gradually reduces as they get older. To find out why sleep changes as the brain develops, Van Savage at the University of California, Los Angeles, and his colleagues collected published data on brain activity and size and sleep duration across different age groups.
They used this information to build a model of how these different aspects might be expected to change as we grow. This allowed them to swap in different figures to test various ideas. For example, if the brain is learning during rapid eye movement (REM) sleep, this would lead to a prediction that the duration of REM sleep is linked to aspects of brain development, which can then be tested against published findings.
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The group found that most of the brain processes associated with learning occur during REM sleep, and that this appears to be the most important function of sleep generally in young infants, who get much more REM sleep than adults.
But there seems to be an abrupt shift in toddlers. 鈥淏efore two-and-a-half, sleep is mainly about鈥� rewiring the brain to learn and grow,鈥� says Savage. But after this age, the main function of sleep appears to be the repair of any damage to the brain. 鈥淚 was surprised that it was such a sharp transition point,鈥� says Savage, who likens the sudden change to water freezing into ice.
鈥淭here are tonnes of other things that are happening around this time,鈥� says Marcos Frank at Washington State University. It is about this age that children start sleeping for longer throughout the night, for example, and the development of multiple systems, including vision and language, is well under way, he says.
鈥淲e know that that is when the brain goes from an age of 鈥榖uild, build, build鈥� to 鈥榟oning and clean-up鈥�,鈥� says Rebecca Spencer at the University of Massachusetts Amherst. 鈥淚t becomes more about quality rather than quantity in terms of the neural circuitry.鈥� But the findings need to be confirmed by studying how children鈥檚 brains change over time, she adds.
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