A team of scientists has discovered a dramatic change in the purpose of sleep, which takes place when humans reach roughly two-and-a-half years of age, switching from rapid growth to a permanent damage-control function.
Before this milestone, the brain grows very rapidly, making use of Rapid Eye Movement (REM) sleep to build and strengthen synapses, the connections between neurons in our brains, as babies learn at an extraordinary rate.
Once the two-and-a-half-year threshold is passed, however, the researchers now say that sleep’s primary function permanently switches to one of maintenance and repair.
All animals experience ongoing background brain damage as a simple consequence of being alive. This low-level degradation results in debris, in the form of damaged genes and proteins, which can accumulate over time and cause brain disease in later life.
Sleep is the necessary mechanism used to help clear this debris out and nearly all of this maintenance occurs during sleep, according to senior author Van Savage and the team of boffins from the University of California, Los Angeles.
“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.”
Savage and a cross-disciplinary team of neuroscientists, biologists, statisticians and physicists took data from over 60 sleep studies involving both humans and other mammals and examined the impact of factors such as total sleep time, total REM sleep time, the brain’s metabolic rate, and brain size relative to body size.
Their findings were published in the journal Science Advances.
The team collected all the data from each of the 60 studies and used it to build and test a mathematical model to examine the function of sleep over time and the changes that take place therein.
Across all species tested, including rabbits, rats and pigs, the results were uniform: a dramatic decline in REM sleep when they reached the developmental age equivalent of two-and-a-half human years.
The researchers noted an inverse relationship between brain growth and the amount of REM sleep; as we age and our brains develop, we get less and less REM sleep.
For example, in newborns, roughly 50 percent of their sleep is REM sleep, which facilitates extraordinarily fast brain growth. However, in 10 year-olds, REM sleep drops to roughly 25 percent of total sleep, whereas in adults over 50, this drops to just 15 percent of sleeping time.
“I fought sleep and pulled all-nighters when I was in college, and now think that was a mistake,” Savage said. “I would have been better off with a good night’s sleep. Now when I feel tired, I don’t have any guilt about sleeping.”
The researchers advocate sleeping when needed, to stave off potential problems with brain disorders in later life such as dementia and other cognitive disorders, diabetes, and obesity, among others.
However, some have expressed a degree of skepticism about the fresh research. Jerome Siegel, who studies REM sleep in mammals and was not involved in the research, argues that the UCLA-led team failed to consider factors such as day length, diet and climate, all of which can impact the sleep patterns of humans and other mammals, disputing the accuracy of the study’s findings, citing a dearth of more complete data.
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