Sleep builds the brain in the early years, then maintains it

• During REM sleep, lasting 50% of sleeping time, a baby's brain builds and strengthens synapses
• After 50, adults spend only about 15% of their sleeping time in REM sleep 
• A good night's sleep is important for everyone to keep the brain in good repair

For the very young, sleep builds and strengthens the brain, but it quickly switches to maintenance and repair before a child turns three, new research shows.

Before about the age of 2½, the brain grows rapidly. And during REM (rapid eye movement) sleep a baby's brain builds and strengthens synapses, which connect neurons to each other so they can communicate.

After that, sleep's main purpose switches to preserving function, and this role continues for the rest of your life, according to the study.

"Don't wake babies up during REM sleep – important work is being done in their brains as they sleep," said study co-senior author Gina Poe. She's a professor of integrative biology and physiology at the University of California, Los Angeles (UCLA).

Keeping the brain in good repair

Van Savage is study co-senior author and a professor of ecology and evolutionary biology and computational medicine at UCLA. He said, "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. It's a transition that is analogous to when water freezes to ice."

The research team's analysis of dozens of sleep studies found that REM sleep – when vivid dreams occur – decreases with growth in brain size.

Newborns spend about 50% of their sleep time in REM sleep, but that falls to about 25% by the age of 10 years and continues to decrease with age. Adults older than 50 spend about 15% of their time asleep in REM sleep, the study authors said in a university news release.

Adults have five REM cycles during a full night of sleep (about 7½ hours) and can have a few dreams in each cycle, according to the researchers.

They noted that a good night's sleep helps keep the brain in good repair.

"Sleep is as important as food," Poe said. "And it's miraculous how well sleep matches the needs of our nervous system."

The findings were published in September in the journal Science Advances.

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