Why We Sleep
The Roles REM and NREM Sleep Cycles Play in Maintaining Healthy Cognitive Functioning
In “Why We Sleep,” by Matthew Walker, sleep is considered a pivotal biological mechanism that aids in memory retention, skill acquisition, and physical recovery. Two particular sleep stages, REM and and NREM, play a significant role in the processes listed above and the more a person gets deep sleep, the more likely they are in receiving the benefits afforded by both cycles. Walker also details the consequences incurred from sleep deprivation, which includes microsleep, emotional irrationality, and an increased risk of Alzheimer's.
Biologically speaking, sleep cycles are defined as either (NREM) non rapid eye movement or (REM) rapid eye movement sleep. Both sleep cycles alternate every 90 minutes or so while we are sleeping, with the majority of NREM dominating the first few hours of sleep, and REM the latter end of our sleep. One reason as to why this asymmetric pattern exists reports that NREM sleep exists to “identify which memories are fresh and salient, and which memories are overlapping, redundant or simply no longer relevant.” In other words, while we are experiencing NREM sleep, unnecessary neural connections are weeded out. This is a necessary component in memory consolidation in that we have a finite storage capacity. Only neural connections that are important are kept and strengthened.
Unlike NREM sleep, which is defined by slow wave sleep, REM sleep operates at higher frequency brain waves and perfectly mimics brainwave activity seen in alert people. REM sleep allows sensory signals to travel from the thalamus to the cortex, where “signals of emotions, motivations, and memories (past and present) are all played out in the big screens of our visual, auditory, and kinesthetic sensory cortices in the brain.” This information processing involving different sensory cortices allows for integration, or more specifically, for the interconnecting of raw experiences that help us in gaining insight and problem solving strategies when it comes to the real world. Whereas NREM sleep works as a memory courier that consolidates memories into long-term storage, REM sleep integrates these new memories as important information pieces to be used later.
The benefits of deep sleep, which includes alternating cycles of REM and NREM, are best seen in the acquisition of different types of memory skills. Sleeping before learning increases the brain's ability to make new memories. This is possible because during sleep, memories held in a short-term reservoir (the hippocampus) get transferred to the long-term reservoir (the cortex). This sense of “memory refreshment” is related to NREM sleep and the more short powerful bursts of electrical activity experienced during it directly relates to a greater “restoration of one's learning ability.”
Another memory skill called consolidation is strengthened when learning is followed by sleep. When a person experiences NREM sleep they are more than 20 to 40% likely to retain more information than a person who stays up and forgoes deep sleep. This challenges a lot of conventional thinking around studying, which argues that forgoing sleep to study is more impactful than sleeping more. However, researchers have time and time again proven that consolidation formed during sleep is much more potent than trying to retain more knowledge by staying up. In fact, even daytime naps as short as 20 minutes have been proven to give a memory consolidation advantage. In short, sleep before and after learning incurs much more benefits when it comes to memory retention and consolidation.
Sleep deprivation, or the absence of sleep, has likewise been linked to acute memory lapses, emotional problems, and life-changing illnesses. Research has proven that sleep deprivation, whether in the case of sleeping for only 4 hours or having a single 72 hours of no sleep causes microsleeps: a condition that occurs when a person loses consciousness along with motor control for a brief moment. More worrisome is the fact that when sleep deprivation occurs for longer periods of time (ex: 3 days with only 4 hours of sleep) the rate at which microsleep occurs increases by 400%. This is especially dangerous for people who are performing a test that requires total concentration (driving, operating machinery, performing surgery).
Key findings have also shown that emotional regulation becomes much harder after restricted or no sleep during a full night of sleep. When a person has a full 7-8 hours of deep sleep, the prefrontal cortex (which is related to rational, logical thought and decision making) is perfectly in sync with the amygdala (which is linked to strong emotions and the fight or flight response). Brain scans on sleep deprived patients show hyperactivity in the amygdala and less activity in the prefrontal cortex. In other words, the relationship between the “emotional gas pedal” (amygdala) and the “break” (prefrontal cortex) was mirrored by stronger emotional responses with less inhibitory control. Participants in this case showed much more severe mood swings: their moods often went from euphoric to extremely negative. The link between sleep deprivation and emotional dysregulation was most prevalent in adolescents, who were more likely to be suicidal, aggressive, or violent when sleep deprived. On the flip side, improved sleep has been proven to have therapeutic effects on those with psychiatric illnesses including depression, bipolar disorder, anxiety, and suicidal behavior.
Sleep deprivation has also been linked to Alzheimer's disease, which is a type of dementia marked by memory loss. According to a study done by doctor Maiken Nedergard, a kind of “sewage system” called the glymphatic system exists in our brains. During waking hours, metabolic contaminants created by neurons buildup and cells near neurons called glial cells work to remove dangerous contaminants from the body. It is during NREM sleep, however, that glial cells are most effective in flushing out metabolic contaminants. During this period of deep sleep, glial cells shrink in size in order to let cerebral spinal fluid in the brain flush out metabolic waste created from neural activity during the day. In patients with Alzheimer's, a poisonous protein called beta-amyloid builds up into plaques around the frontal lobe of the brain. Without deep NREM sleep, amyloids build up in deep sleep generating regions and as sleep gets affected, more amyloids build up. This direct link between sleep deficiency and Alzheimer's, along with the risks posed by microsleeps and emotional dysregulation from poor sleep, points to the importance of prioritizing sleep in one's life.
The psychological effects of sleep deprivation are damaging. From loss of consciousness to impaired cognitive ability, losing deep sleep affects all aspects of life. These include everyday mundane activities to activities that require complete focus. Likewise, proper sleep with enough REM and NREM cycles provides therapeutic benefits in the form of emotional regulation and decreasing the likelihood of developing degenerative brain diseases. The common misconception linking laziness with sleeping needs to be addressed in order for society to fully understand and acknowledge the risks they take when they choose to forgo sleep.
