UCLA-led team of scientists discovers why we need sleep

September 18, 2020

Prolonged sleep deprivation can lead to severe health problems in humans and other animals. But why is sleep so vital to our health? A UCLA-led team of scientists has answered this question and shown for the first time that a dramatic change in the purpose of sleep occurs at the age of about 2-and-a-half.

Before that age, the brain grows very rapidly. During REM sleep, when vivid dreams occur, the young brain is busy building and strengthening synapses -- the structures that connect neurons to one another and allow them to communicate.

"Don't wake babies up during REM sleep -- important work is being done in their brains as they sleep," said senior study author Gina Poe, a UCLA professor of integrative biology and physiology who has conducted sleep research for more than 30 years.

After 2-and-a-half years, however, sleep's primary purpose switches from brain building to brain maintenance and repair, a role it maintains for the rest of our lives, the scientists report Sept. 18 in the journal Science Advances. This transition, the researchers say, corresponds to changes in brain development.

All animals naturally experience a certain amount of neurological damage during waking hours, and the resulting debris, including damaged genes and proteins within neurons, can build up and cause brain disease. Sleep helps repair this damage and clear the debris -- essentially decluttering the brain and taking out the trash that can lead to serious illness.

Nearly all of this brain repair occurs during sleep, according to senior author Van Savage, a UCLA professor of ecology and evolutionary biology and of computational medicine, and his colleagues.

"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 research team, which included scientists with expertise in neuroscience, biology, mathematics and statistics, conducted the most comprehensive statistical analysis of sleep to date, using data from more than 60 sleep studies involving humans and other mammals. They examined data on sleep throughout development -- including total sleep time, REM sleep time, brain size and body size -- and built and tested a mathematical model to explain how sleep changes with brain and body size.

The data were remarkably consistent: All species experienced a dramatic decline in REM sleep when they reached the human developmental equivalent of about 2-and-half years of age. The fraction of time spent in REM sleep before and after that point was roughly the same, whether the researchers studied rabbits, rats, pigs or humans.

REM sleep decreases with the growth in brain size throughout development, the scientists found. While newborns spend about 50% of their sleep time in REM sleep, that falls to about 25% by the age of 10 and continues to decrease with age. Adults older than 50 spend approximately 15% of their time asleep in REM. The significant dropoff in REM sleep at about 2-and-a-half happens just as the major change in the function of sleep occurs, Poe said.

"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."

A chronic lack of sleep likely contributes to long-term health problems such as dementia and other cognitive disorders, diabetes, and obesity, to name a few, Poe said. 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," 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."

For most adults, a regular seven-and-a-half hours of sleep a night is normal -- and time lying awake doesn't count, Poe says. While children need more sleep, babies need much more, roughly twice as much as adults. The large percentage of REM sleep in babies is in stark contrast to the amount of REM sleep observed in adult mammals across an enormous range of brain sizes and body sizes. Adult humans have five REM cycles during a full night of sleep and can have a few dreams in each cycle.

A good night's sleep is excellent medicine, Poe says. And it's free.
-end-
Co-authors of the study are 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.

Funding sources included the National Science Foundation and the Eugene and Clare Thaw Charitable Trust.

University of California - Los Angeles

Related Sleep Articles from Brightsurf:

Size and sleep: New research reveals why little things sleep longer
Using data from humans and other mammals, a team of scientists including researchers from the Santa Fe Institute has developed one of the first quantitative models that explains why sleep times across species and during development decrease as brains get bigger.

Wind turbine noise affects dream sleep and perceived sleep restoration
Wind turbine noise (WTN) influences people's perception of the restorative effects of sleep, and also has a small but significant effect on dream sleep, otherwise known as REM (rapid eye movement) sleep, a study at the University of Gothenburg, Sweden, shows.

To sleep deeply: The brainstem neurons that regulate non-REM sleep
University of Tsukuba researchers identified neurons that promote non-REM sleep in the brainstem in mice.

Chronic opioid therapy can disrupt sleep, increase risk of sleep disorders
Patients and medical providers should be aware that chronic opioid use can interfere with sleep by reducing sleep efficiency and increasing the risk of sleep-disordered breathing, according to a position statement from the American Academy of Sleep Medicine.

'Short sleep' gene prevents memory deficits associated with sleep deprivation
The UCSF scientists who identified the two known human genes that promote 'natural short sleep' -- nightly sleep that lasts just four to six hours but leaves people feeling well-rested -- have now discovered a third, and it's also the first gene that's ever been shown to prevent the memory deficits that normally accompany sleep deprivation.

Short sleep duration and sleep variability blunt weight loss
High sleep variability and short sleep duration are associated with difficulties in losing weight and body fat.

Nurses have an increased risk of sleep disorders and sleep deprivation
According to preliminary results of a new study, there is a high prevalence of insufficient sleep and symptoms of common sleep disorders among medical center nurses.

Common sleep myths compromise good sleep and health
People often say they can get by on five or fewer hours of sleep, that snoring is harmless, and that having a drink helps you to fall asleep.

Sleep tight! Researchers identify the beneficial role of sleep
Why do animals sleep? Why do humans 'waste' a third of their lives sleeping?

Does extra sleep on the weekends repay your sleep debt? No, researchers say
Insufficient sleep and untreated sleep disorders put people at increased risk for metabolic problems, including obesity and diabetes.

Read More: Sleep News and Sleep Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.