Why Do Humans Need Sleep? It Depends On Our Age

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Researchers have known for decades that sleep is crucial for our brains, but its main function dramatically shifts depending on our age, according to a new study published in Science Advances.

Even during sleep, our brain remains busy. The leading hypothesis is that during the hours we spend in the arms of Morpheus, our brain is either repairing itself and getting rid of toxins or it is learning and improving. To investigate further, the team used data – such as total sleep time, REM sleep time, the volume of white matter, and brain size – from more than 60 studies to determine whether either hypothesis was dominant. 

The team found that a major change in the purpose of sleep happens at 2.4 years of age. Up until that point, the brains of babies and toddlers use REM sleep to reorganize itself. After that, non-REM sleep becomes dominant. The scientists believe that’s when the brain's main job during sleep is to clean and repair itself.

"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," senior author Van Savage, a professor of ecology and evolutionary biology and of computational medicine from UCLA, said in a statement. "It's a transition that is analogous to when water freezes to ice."

Newborns sleep for about 50 percent of this time in the REM phase, according to the authors. By the time they are 10, only a quarter of their sleep is REM and it continues to diminish with age. Adults older than 50 only spend 15 percent of their sleep in REM.

"Sleep is as important as food," senior co-author Gina Poe, a UCLA professor of integrative biology and physiology, explained. "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."

The team collected data from both human and animal studies, and while there are some interesting similarities between us and other mammals, the team believes more data is needed to better understand the evolutionary push for this brain activity during sleep.

"I'm very interested to see if our framework also can be extended to other mammals," explained lead author Dr Junyu Cao, from the University of Texas at Austin.

Current research shows that chronic lack of sleep plays a role in cognitive disorders, dementia, diabetes, and obesity.

"I fought sleep and pulled all-nighters when I was in college, and now think that was a mistake," Savage added. "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."

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