A dramatic change in the purpose of sleep occurs when children are about 2 1/2 years old -- a time when sleep's primary purpose changes from brain-building to brain maintenance and repair, according to a study released Friday by researchers at UCLA.

Don't wake babies up during REM (rapid eye movement) sleep. Important work is being done in their brains as they sleep," said Gina Poe, the senior study author and a UCLA professor of integrative biology and physiology who has conducted sleep research for more than three decades.

Newborns spend about 50% of their sleep time in REM sleep, with that number falling to about 25% by the age of 10 and continuing to decrease with age. Adults who are older than 50 spend about 15% of their time asleep in REM, according to researchers.

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The study, published in the journal Science Advances, noted that the sharp transition in sleep function is ``remarkable given that this shift likely signals a profound shift in the function of sleep and the behavior of sleep processes.

Researchers, who used data from more than 60 sleep studies involving humans and other mammals, found that all species experienced a dramatic decline in REM sleep when they reached the human developmental equivalent of about 2 1/2 years of age.

The transition at about age 2 1/2 corresponds to changes in brain development, according to researchers, who say that sleep then helps repair a certain amount of neurological damage suffered during waking hours, and essentially declutter the brain.

Sleep is as important as food, Poe said. And it's miraculous how well sleep matches the needs of our nervous system. From jellyfish to birds to whales, everyone sleeps. While we sleep, our brains are not resting.

Poe noted that a chronic lack of sleep likely contributes to long-term health problems such as dementia and other cognitive disorders, and urged people to go to bed when they start to feel tired.

Nearly all of the brain repair occurs during sleep, according to the study's senior author, Van Savage, a UCLA professor of ecology and evolutionary biology and of computational medicine.

I was shocked how huge a change this is over a short period of time, and that this switch occurs when we're so young, Savage said. It's a transition that is analogous to when water freezes to ice.

The study was co-authored by Junyu Cao, who conducted research in Savage's laboratory and is now an assistant professor at the University of Texas at Austin; Alexander Herman, an assistant professor of psychiatry at the University of Minnesota, Twin Cities; and Geoffrey West, a physicist who is the Shannan Distinguished Professor at the Santa Fe Institute.

The National Science Foundation and the Eugene and Clare Thaw Charitable Trust helped to fund the study.

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New UCLA Research Reveals Why Sleeping is So Important - NBC Southern California