Scientists solve the mystery of why we need sleep
In the first couple years of life, sleep helps build the brain’s infrastructure. After the age of two and a half, sleep takes on an entirely new decluttering role, according to a new study from UCLA.
While it is known that sleep deprivation can lead to severe health problems, much about the purpose and function of rest has remained a mystery. The UCLA team has discovered for the very first time that the purpose of sleep takes a dramatic shift early in life.
Before the age of 2.5 years, the brain grows very rapidly. As vivid dreams occur during the REM phase, the young brain is building and strengthening synapses, which are the structures that connect brain cells together so they can communicate.
“Don’t wake babies up during REM sleep – important work is being done in their brains as they sleep,” said senior study author Professor Gina Poe, who has conducted sleep research for more than 30 years.
The researchers report that after the age of 2.5, the primary purpose of rest switches from building the brain to maintaining and repairing it, and this becomes its permanent role.
A certain amount of neurological damage naturally accumulates during waking hours, including damage to genes and proteins within neurons. Over time, this debris can build up and cause brain disease. Sleep helps repair this damage, essentially decluttering the brain and eliminating the debris that can lead to serious illness.
According to study senior author Professor Van Savage, nearly all of this brain repair occurs during sleep. “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.”
For the investigation, the researchers conducted the most comprehensive statistical analysis of sleep to date. The analysis was focused on data from more than 60 studies involving humans and other mammals.
The evidence was very consistent that all species experience a dramatic decline in REM sleep when they reach the human developmental equivalent of about 2.5 years of age. The fraction of time spent in the REM phase after this point was found to be roughly the same across all humans and animals studied.
While newborns spend about 50 percent of their rest time in the REM phase, this drops to 25 percent by age 10 and continues to decline with age. The significant dropoff in REM sleep at about 2-and-a-half happens just as the major change in the function of sleep occurs, said Professor Poe.
“Sleep is as important as food. 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.”
Chronic sleep deprivation likely contributes to long-term health problems such as dementia and other cognitive disorders, diabetes, and obesity, to name a few, said Professor Poe. When you start to feel tired, she said, don’t fight it – go to bed.
“I fought sleep and pulled all-nighters when I was in college, and now think that was a mistake,” said Professor Savage. “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.”
According to Professor Poe, a regular seven-and-a-half hours of rest a night is normal, and time lying awake doesn’t count. “A good night’s sleep is excellent medicine. And it’s free.”
The study is published in the journal Science Advances.