Why do we sleep? Study shows how our brains work tirelessly from birth to old age while we rest

LOS ANGELES, Calif. — Everyone sleeps, but have you ever wondered why we need so much slumber? Researchers at the University of California, Los Angeles (UCLA) have found the answer. Their study says the brains of humans and other mammals are hard at work while you rest, growing and repairing systems. For babies, sleep is even more important and can make or break their development.

“Sleep is as important as food,” says study co-author Gina Poe in a university release. “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.”

Researchers note that before about two-and-a-half years of age, the human brain is developing rapidly, especially during times of rapid eye movement (REM) sleep. During this time of vivid dreams, the brain functions like a construction crew building synapses that will allow neurons to connect and communicate with one another.

“Don’t wake babies up during REM sleep,” warns the UCLA professor of Integrative Biology and Physiology. “Important work is being done in their brains as they sleep.”

A baby’s sleep is vital to their brain development

The research team studied data from more than 60 sleep studies on humans and other mammals to complete what may be the most thorough statistical analysis of sleep to date. Part of their study involved building and testing a mathematical model to demonstrate the ways sleep changes along with brain and body size.

Scientists found that once humans reach two-and-a-half the primary structure of the brain has been built and there is a shift in the function of sleep. From this point on, sleep becomes more about brain maintenance and repair than about construction. This shift, researchers say, goes along with changes in brain development.

What surprised scientists most was the uniformity of data results across all species. A startling drop in REM sleep was seen in all species at the same equivalent developmental stage as a human toddler. The percentage of time spent in REM sleep before and after this brain-equivalent age was the same, whether human, pig, rabbit, or rat.

“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,” reports study co-author and UCLA professor Van Savage. “It’s a transition that is analogous to when water freezes to ice.”

Dream sleep is less important when you’re older

It is no wonder that babies need so much sleep. Researchers say that while adults do fine on 7.5 hours of sleep, babies need roughly twice that amount to accomplish all that needs to happen during REM sleep.

The study results found that the decreases in REM sleep are in direct proportion to the increases in brain growth. A newborn spends about 50 percent of sleep time in REM sleep. This drops to about 25 percent of sleep time by age 10 and continues to decrease with age. By the time adults reach the age of 50, REM sleep falls off to just 15 percent of sleep time. The biggest drop in REM sleep occurs at the exact same time the purpose for sleep changes.

Scientists also point out that sleep is important for combating the neurological damage that occurs during waking hours. Researchers report that sleep is the period when almost all brain repairs take place. Damaged genes and proteins within neurons that could lead to brain disease get cleared out while humans get their rest.

“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,” says Poe. “When you start to feel tired, don’t fight it–go to bed.”

According to legend, Rip Van Winkle slept for 20 years and woke up to a different world. Tempting as that might sound in 2020, it’s probably best to take Poe’s advice of a sleep expert: “A good night’s sleep is excellent medicine… And it’s free.”

Findings are published in the journal Science Advances.

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