Dream Sleep Helps Older and Younger Brains in Different Ways

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Just before the age of 2-and-a-half, rapid eye movement (REM) sleep's primary function is to build the brain's infrastructure by strengthening or pruning experience-dependent synapses; this neural reorganization occurs when babies' brains have especially robust neuroplasticity.

However, like clockwork, a recent analysis of data from over 60 sleep studies involving both humans and other mammals suggests that after the equivalent age of about 2.4-years-old, sleep's primary purpose abruptly switches from "infrastructure-building and synapse-strengthening" mode to helping the brain declutter by facilitating debris-removing, reparative functions.

The findings of this UCLA-led quantitative analysis (Cao et al., 2020) were published on Sept. 18 in the journal Science Advances. First author, Junyu Cao, is currently an assistant professor at the University of Texas at Austin. According to the authors—which included a diverse team of scientists with expertise in biology, neuroscience, statistics, and mathematics—this is one of the most comprehensive quantitative analyses designed to unravel the age-old mystery of why we need sleep.

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REM Sleep Is Also Referred to as "Dream Sleep"

The REM sleep cycle is when humans and animals dream. Anyone who's ever watched your pet dog sleeping during a REM cycle may have observed your dog's eyes darting back and forth beneath closed lids.

One of the first known written observations of REM sleep was not of a sleeping human, but rather of someone's pet dog. Over 2,000 years ago, a Roman poet and philosopher, Lucretius (c. 99—c. 55 B.C.E.), was watching his dog sleeping by a fire and noted that "the animal appeared to be chasing some phantom prey in its mind." As the dog's eyes darted back and forth and its legs twitched to emulate a running motion, Lucretius speculated that the animal was having a vivid dream.

Surprisingly, it wasn't until the 1950s that modern-day scientists (Aserinsky & Kleitman, 1953) finally investigated the possible link between human REM sleep and dreaming. Eugene Aserinsky and Nathaniel Kleitman of the University of Chicago were fascinated by regularly occurring periods of "rapid, jerky, and binocularly symmetrical" eye movement during sleep and conducted human studies to "confirm the conjecture that this particular eye activity was associated with dreaming."

Because the majority of our vivid dreams occur during rapid eye movement sleep, many people refer to REM sleep as "dream sleep." That said, it's important to note that recent studies suggest that some dreaming also happens during non-rapid eye movement (NREM) sleep (Siclari et al., 2017).

REM sleep occurs in cycles throughout the night. During a seven- to eight-hour night of sleep, healthy adults typically have about four REM cycles. But as we age, humans tend to spend less and less time in REM. For example, older adults (over age 50) typically spend about 15 percent of each night's sleep in REM sleep, whereas newborn babies evenly divide their 10.5 to 18 hours of sleep per day between REM and non-REM sleep in a 50/50 split. By the age of 10, most children spend about 25 percent of their sleep time in REM dream sleep.

In the discussion section of their latest (2020) quantitative analysis, Cao et al. sum up the significance of their conclusion that both human and mammalian brains experience an abrupt transition from neural reorganization to repair between 2 to 3 years of age:

"Our new theory, mathematical models, and data analysis provide compelling evidence that these fundamental differences arise because sleep is used primarily for neural reorganization until about 2 to 3 years of age, at which point, there is a critical transition, and the function shifts sharply toward sleep being for repair and clearance. We identify the specific turning point as occurring at an unexpectedly precise age of around 2.4 years old, reflecting a critical physiological or cerebral developmental change."

Interestingly, across species, this quantitative analysis of REM sleep cycles found that humans, rabbits, rats, and pigs all experience a dramatic decline in REM sleep when they reach the human developmental equivalent of roughly 2-and-a-half years of age.

Along this line, last year, researchers at Boston University (Fultz et al., 2019) identified how cerebrospinal fluid (CSF) washes in and out of the brain during NREM sleep. As this fluid pulsates out of the cerebrum, it flushes away debris. As part of this nightly "brainwashing," pulsing waves of CSF wash away toxic proteins and metabolic waste that can damage the brain. (See "Flushing the Brain While You Sleep" by William Klemm)

In a Sept. 18 news release, the UCLA-based senior authors of the recent (2020) quantitative analysis emphasize that because all animals experience small amounts of debris-producing neurological damage during waking hours, sleep plays a vital housekeeping role akin to 'taking out the trash.' "Nearly all of this brain repair occurs during sleep," co-senior author Van Savage, a UCLA professor of ecology, evolutionary biology, and biomathematics, said in the news release.

"Don't wake babies up during REM sleep—important work is being done in their brains as they sleep," co-senior author Gina Poe, professor of integrative biology and physiology at UCLA, advises. "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."