Neuroscience of Sleep
Leora Sazant

In mammals, the overall sleep cycle is comprised of two broad alternating cycles, REM (rapid eye movement), and NREM (non-rapid eye movement), which consists of three individual phases[1] . Each alternating cycle is approximately 90-110 minutes in duration, and occurs in succession throughout sleep; however, REM tends to increase in duration as the cycle progresses throughout sleep [1] . The quality and duration of these cycles are of utter importance when considering memory consolidation and learning, activation of the visual system, gender- and sex-related differences, and other important physiological processes. Activation of different areas of the brain are said to be involved in REM and NREM sleep; the limbic system and amygdala are highly activated during REM, whereas more cortical areas are activated during NREM [1] . Although it was previously thought that a majority of dreaming occurs during REM sleep, new research using functional brain imaging is now proposing otherwise. In fact, there are now studies showing that different emotions and feelings are elicited throughout the different stages of sleep, and are thought to have an impact social interactions as well as overall mood during the awake state.



3.1 REM Sleep


Rapid eye movement sleep (known as REM sleep) consists of a smaller portion of the overall sleep
300px-REM.png
Figure 1. A sample EEG of REM sleep. It is characterized by low amplitude and high frequency pattern.

cycles (approximately 25%), yet it has several unique characteristics that distinguish it from the other sleep stages[2] . It is known for its low amplitude and high frequency EEG pattern[3] .

Functions

Current research proposes a few different theories that try to explain the functions of REM sleep and the reasons why it is so important. All of the proposed theories indicate that this stage of sleep is critical for many processes such as memory consolidation and learning, as well as visual system processing.

The Role of REM in Memory Consolidation and Learning

It was first proposed that REM sleep had a vital function in the role of memory consolidation and types of non-declarative memory. In fact, in early studies that tried to prove this theory, researchers investigated the difference in memory consolidation during REM sleep and REM sleep deprivation (REMD)[4] . In these studies, the researcher would record EEG activity during NREM sleep, and then wake the subject up during their first signs of REM sleep[2] . Previous results suggested that REM sleep was important in the role of procedural memory; that is, memory important for knowing how to do and perform things (a type of implicit memory). However, this was clearly not very advantageous, because it was quite a limited approach.

New research has been conducted to further investigate this theory of the significance of REM sleep in memory and learning. In a study done by (10), REM sleep was suppressed by administering certain pharmaceutical inhibitors, such as noradrenaline and selective serotonin re-uptake inhibitors(SSRIs)[3] . Due to the fact that the REM sleep pattern consists of a large amount of serotoninergic and norepinephrinergic neurotransmitter activity, inhibiting these neurotransmitters would therefore disrupt the REM cycle[5] . Afterwards, the researchers measured skill-memory responses to see if a lack of REM had any significant impact on the procedural aspect of non-declarative memory. After performing the experiment, the results indicated that inhibiting REM had no significant impact on this type of memory system[3] . Thus, it could be suggested that REM sleep is not essential for procedural and skill-related memory consolidation, contrary to previous beliefs.

Stimulation/Activation of the Visual System

Other results from current research is now also proposing that REM sleep may have an important role in the development of the visual system[6] . Specifically, REM sleep may be of vital significance during mature development of the visual cortex, in mammals such as kittens[7] . Researchers further investigated this hypothesis by inhibiting responsiveness in one eye, by performing monocular deprivation – surgically closing one of the eyes[5] . In doing so, this limits the amount of neuronal projection to the cortex, which resembles REM sleep[3] . The findings suggested that the cells in the visual cortex of the occluded eye – the lateral geniculate cells – were reduced in size in those with REM sleep deprivation[3] .

REM in Children with Attention Deficit Hyperactivity Disorder

results_1.jpg
New studies are now discovering that there seems to be an increase in the amount of time spent in REM sleep in children with attention-deficit/hyperactive disorder (known as ADHD)[8] . To further investigate the effect that this may have on cognitive development and neurobehavioral functions, researchers explored the differences in a two-night polysomnography of children with ADHD and without ADHD. In addition to this, tests were distributed to these children, which measured their neurobehavioral and cognitive functions. After performing the experiment, the results suggested that children with ADHD spent significantly more time in REM sleep, compared to children who did not have ADHD[6] . Furthermore, it was also discovered that this increase in REM sleep duration negatively correlated with IQ intelligence scores[6] . It was also found that there was a positive correlation with CBCL attention problem scores and time spent in REM sleep[6] .

Other research on this topic has explored the neurodevelopment basis for the previous results. In a study completed by Grissom, Brubaker, Capdevila, Hawley, and Gozel (2009), it was revealed that the REM sleep EEG had high amplitudes and low frequency in children with ADHD[7] . In REM, the neurotransmitter dopamine is what influences the movements of the eyes[9] . Thus, the researchers suggest that there may be some sort of problem or deactivation with the dopaminergic pathway in the brain, which could be the reason for this increase in REM sleep in ADHD[7] .

REM Dreams

A majority of dreaming does occur in the REM stage of sleep. However, these dreams only begin to mimic adult-like consciousness after the brain has completely developed
[4] . Researchers are still investigating the exact age when people are able to start to have dreams as well as realize they are having a dream; the ages among debate are between five and eight years of age
[4] . Specifically, when people are woken in the middle of a REM dream, the dream that they often experience having is one that involves unpleasant, bizarre, and implausible events[10] .

Aggressive Impulses in REM Dreams

It is known within the science community, that a decrease in serotoninergic activity has a positive correlation with social aggression in most mammals[11] . As well, REM sleep is characterized by episodic decreases in serotoninergic activity within the forebrain, as well an overall high amygdala activation[11]. Therefore, in a study performed by McNamara, McLaren, Smith, Brown, and Stickgold (2005), information was discovered about the content presented in REM dreams, that related with the idea of a reduction in serotoninergic activity and increase social aggression[12] . ‘Nightcap’ technology was used to observe the data, a tool used to measure the amount of eyelid and head movements within a 250 ms period[9] . Afterwards, the numbers of movements were scored with a variety of standardized test, which measured different social interaction. Specifically, it was found that during REM sleep, more aggressive impulses were stimulated and observed – 65% of the REM reports suggested this[9] . Furthermore, there were accounts of physical aggressiveness that was shown in the REM reports – 25% of the REM reports suggested this[9] .

3.2 NREM Sleep


Definition and Brain Wave Activation

Non-rapid eye movement sleep (known as NREM sleep) is divided into three separate phases, each of which is characterized by 90-100 minute cycles that occur in succession throughout the sleep cycle. It alternates with the REM sleep cycle and consists of a majority of the sleep cycle: 75%[1] . NREM is unique in that it is distinguished by its sleep spindles, K complexes that last approximately 0.5 seconds, and delta wave brain activity, which are all represented in Figure 3[13] . These characteristic delta
sleepwaves.gif
Figure 3. An EEG recording of the different brain waves during sleep. This picture includes the hallmark characteristics of NREM sleep including: K complex, sleep spindles, and delta waves.
waves are the hallmark property of the NREM sleep cycle, consisting of high amplitudes (>75uV) and slow frequency (1-4 Hz)
[10] . Delta waves tend to have the same physical characteristics at all times, however their activity is not entirely uniform. Specifically, the length of time spent in NREM sleep varies throughout; it occurs maximally when sleep begins and lessens as duration of sleep increases
[1] . As well, it is suggested that there is a direct correlation between the length of time spent in NREM sleep and the amount of time spent awake, prior to sleep
[1] . This research shows that recorded delta waves will consist of larger, more abundant amplitudes, if a longer duration is spend during prior awake hours
[1] .


Slow-Wave Sleep

Slow-wave sleep (SWS) is a main characteristic within the NREM sleep cycle, where sleep is spent 10-25% of its total cycle[11] . It is comprised of the third and fourth phase of NREM sleep, and is considered to be the deepest stage of sleep, as well as the most restorative[11] . As well, it is also associated with cerebral restoration and quality of sleep[14] . In fact, if an insufficient amount of time is spent in SWS, it will be made up for in subsequent sleep, but will be detrimental to daytime functioning[11] . Also, SWS is the stage where sleepwalking occurs[2] . This indicates that SWS is a vital part of the sleep cycle. Medications have been produced to enhance the amount of time spent in SWS, which has been said to improve daytime function and overall quality of sleep[10] .

The Role of SWS in Memory and Learning

Recent research suggests that SWS has a role in the consolidation of certain memories and learning processes within the brain. The hallmark characteristics of the NREM sleep stage, including slow oscillations, spindles, and ripples, all help redistribute the memories stored in the hippocampus to cortical sites[2] . This information along with new experimental data, suggest that SWS-rich sleep have a positive impact on the consolidation of declarative, hippocampus-dependent memories (a type of implicit memory)[2] .
Some studies have also investigated how SWS can improve perceptual learning, which is skill-based learning. In an experiment performed by Born, Pommer, Rasch, and Diekelmann (2008), SWS was suppressed and visual texture discrimination was measured (a form of perceptual learning) to determine whether perceptual learning increased as SWS increased[15] . The results from this experiment revealed that improved visual texture discrimination resulted from an increased duration in SWS[12] . Another interesting finding from this study showed that there was an improvement in visual texture discrimination after a 60-90 minute nap that consisted of only SWS. These findings all suggest that SWS has an important role in perceptual learning[12] .

Gender-and Age-Related Differences in SWS

New research has explored the areas of gender- and age-related differences within SWS, and their possible effects. Specifically, there have been some differences that have been observed in middle-aged and older people. In one study, the results indicated that middle-aged and older women tend to have an overall longer duration of SWS during the sleep cycle, than men do[10] . However, another study was conducted that used young men and women as subjects (mean age = 23.5 years). The results from this study revealed that they had similar amounts of SWS sleep[10] . Current research is still investigating what accounts for this gender difference in middle aged and older subjects. This is an important aspect of modern research because SWS deprivation in men and women is a large problem. Particularly, in today’s society, more women have sleep problems such as insomnia[10] .
The effect of age on SWS is another area of recent research. In general, SWS is at its highest intensity and quality during adolescence years, and then it declines as age increases[10] . As well, it is more likely for subjects to wake up in the middle of sleep as age increases[10] . Interestingly enough, these sleep changes occur in almost every individual, even those with the healthiest medical record[10] . Overall, it could be concluded that older individuals who are male have less SWS than younger women who are female.
Current research is now investigating how individual differences in SWS can be predicted from an early age. Furthermore, researchers are exploring the effects of having the adenosine deaminase gene, the circadian clock gene, or those who code for the PER3 protein[10] . This indicates there is a molecular basis that links SWS and circadian rhythms together[10] .

NREM Dreams

It was originally thought that all dreaming occurred in REM sleep. However, new research suggests that this is not the case because a portion of dreaming occurs in NREM sleep as well[4] . Some researchers have actually found that when a person is awoken from a dream in a NREM stage of sleep, they are more likely to report having dreams that are more life-like and familiar, whereas in REM, the opposite seems to be true[8] . This is a very interesting theory. However, it is still in debate because some researchers think that dream states are not as straightforward as this theory proposes[8] . This topic is still undergoing current research.



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