As we drift off to sleep at night, we are probably unaware of the ways in which our good night’s rest will benefit all the organs and systems of the body. One of the most important organs needing sleep is our brain. Sleep is critical for brain function and restoration. The body’s internal housekeeping processes that occur during sleep have a profound impact on brain health, mood, memory, and new learning. Research studies have shown that the brain’s structure and function change during sleep, and that sleep helps us store experiences and ideas while enhancing creativity and our ability to solve problems. If we don’t get a good night’s sleep, the next day we may find ourselves irritable, forgetful, depressed, or even prone to falls or accidents.

In the first article in this series, we looked at ways to improve our sleep. In this second article, we’ll explore how different stages of sleep support the brain, especially in enabling it to cleanse itself of toxins and in facilitating memory and learning.

Stages of Sleep

Over the course of a night, sleep unfolds in cycles, each consisting of five different stages of sleep that alternate throughout the night. In Stage 1, the falling asleep stage, which typically occurs sometime between 9:00 and 11:00 p.m., research indicates that the neurotransmitter serotonin, often called the happiness neurotransmitter, is produced in its highest values. Stage 2, somewhat deeper sleep, has been associated with, among other things, developing and improving motor skills used for activities like playing a musical instrument.

REM sleep plays a role in memory storage and in integrating daily experiences and memories.

The third and fourth stages (often considered to be one stage) are deep sleep, which plays a major role in both brain cleansing and memory. The last, or fifth, stage is known as rapid eye movement (REM) sleep, or dreaming sleep. This stage is characterized by active eye movements and also by body paralysis, which assures that a sleeping person is protected from acting out the dream. REM sleep plays a role in memory storage and in integrating daily experiences and memories.

How the Brain Cleans Itself During Sleep

During sleep, the brain does its housecleaning, much like the nightly cleaning of a busy office building. In a busy office building we wouldn’t want the cleaning crew to be vacuuming and emptying trash while the office workers are trying to do work that requires concentration, such as calculating figures, remembering organizational plans, and creating problem-solving strategies. Typically, the office cleaning crew comes in after the daytime workers are gone. During the day the brain is actively processing and responding to interactions with the external world. All its energy is used for thinking, talking, problem solving, observing, and listening. During sleep, the brain’s energy is redirected to clearing waste products, as well as storing memories and producing chemicals—like serotonin—that support pleasant moods and emotions.

Central to the housekeeping role of sleep is cerebrospinal fluid (CSF), the liquid surrounding the brain and spinal cord that moves through the brain via channels near blood vessels. A recent study describes how CSF is involved in the deep cleansing of the brain that happens during sleep: In deep sleep, electrical signals known as slow waves appear in the brain right before a wave of CSF appears that washes through the brain, presumably removing toxins. This happens about every 20 seconds. “This electrical wave always happens first, and the CSF wave always seems to follow seconds later. We’ve discovered there are really large waves of CSF that appear in the brain only during sleep,” says study coauthor Dr. Laura Lewis of Boston University. It is these waves during deep sleep that rid our brain of trash.

An important waste product that CSF clears from the brain is a protein called β-amyloid, which is associated with Alzheimer’s disease. One research study showed that amyloid accumulation in the human brain can occur after just one night of sleep deprivation. There is ongoing research on sleep’s impact on β-amyloid accumulation as a risk factor for Alzheimer’s disease.

Sleep, Memory, and Learning

Our brain takes in and briefly houses huge amounts of information, but only a small part of the data is stored for long-term retrieval. Research tells us that the long-term storage (consolidation) of memory happens both during the brain’s slow-wave electrical activity in deep sleep and during REM sleep. Memory is key to new learning.

At some point in our lives, we may have tried an all-night study session, but research shows sleep deprivation reduces the amount of material we can retain by as much as 40%. Research also shows that memory-related performance improves right after a period of sleep. One study showed children’s performance on memory tests improved 14-25% following a period of sleep, whereas little improvement was found following an equal period of wakefulness, though in both cases the children were well rested.

Getting a good night’s sleep is one of the best things we can do for our brain.

A Harvard study showed a 30% improvement in performance on anagram word puzzles after participants had a cycle of REM sleep. As the Harvard study notes, REM sleep is associated with memory-related creative processes and abstract reasoning. Research also suggests that REM sleep plays a major role in consolidating memories that have emotional content, as opposed to those that are emotionally neutral. Experiences and memories of the day can be creatively blended during REM-sleep dreaming.

Sleep disruption often increases as we age, which can contribute to structural changes in the brain that tend to happen with age. Such structural changes can reduce the slow-wave activity of deep sleep, causing communication problems between the brain’s two important memory centers—the prefrontal cortex and the hippocampus—and may also involve a thinning of tissues in these memory centers, all of which can make it harder for short-term memories to be transformed into long-term memories, thus causing long-term memory impairment. The good news is that mind-body practices like meditation help preserve critical brain areas associated with memory. In one study, for example, that showed less cortical thinning in meditators, significant areas of the prefrontal cortexes of meditators age 40–50 had the same amount of gray matter as those in a control group of non-meditators age 20–30!

Getting a good night’s sleep is one of the best things we can do for our brain, but this is not the last thing I want to say about sleep and our health. Sleep is also key to cultivating a healthy immune system, and in the next article we will look at sleep’s role in fighting infections and maintaining an optimal state of wellness.

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