Sleep Stages and Brain Hemispheric Dominance Recorded by EEG and MEG

Sarah Dong


Sleep is broken down into stages corresponding with differing hemispheric dominance in the brain, the stages including non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM is considered to be deep sleep, while REM is a sort of paradoxical sleep, imitating the mental state of that of a waking individual [1]. It has been found that the left hemisphere is predominantly active during NREM sleep, while the right hemisphere is dominant during REM sleep [2]. Electroencephalograms (EEG) and magnetoencephalography (MEG) are often used to collect brain data for research and are especially useful in sleep data, producing visualizations that depict brain activity that allow for insight into how circadian and ultradian rhythms affect individuals’ lives. 


Relationship Between Sleep Stages and Brain Hemispheres

A normal sleep cycle oscillates between waking, REM, and various NREM stages. NREM can be further broken down into NREM 1, 2, 3, and 4. REM, termed as rapid eye movement sleep, is exactly that. REM sleep is where most dreams happen, thus resulting in high frequencies of brain waves being detected and in its striking similnearities to the waking brain state. Deep sleep, especially during NREM stage 4, is a period of dreamless sleep characterized by large amounts of slow delta, low delta, and theta brain waves. REM sleep, on the other hand, is characterized by large amounts of high-frequency alpha, beta, and theta brain waves [3]. The brain waves from fastest (low amplitude) to slowest (high amplitude) brain waves are gamma, beta, alpha, theta, and delta [4]. Thus, lower frequency waves are more commonly found during deep sleep where there is not much brain activity.

As expected, there are various discrepancies between the two hemispheres, especially during differing sleep stages. REM sleep is known to be the main contributor to the mind’s consolidation and transformation of memory, and the brain’s left hemisphere is found to be mainly active during states of wakefulness or REM sleep [5]. Conversely, the right hemisphere is mainly correlated with NREM 4 and related deep sleep stages, underlining the different functions of the hemispheres.


EEG and MEG Functions in Sleep

Electroencephalograms, otherwise known as EEGs, are technological devices that measure electrical activity from organisms’ brains. Cortical pyramidal neurons in the brain’s cerebral cortex that are perpendicular to the brain’s surface radiate off neural activity that electrodes placed around the scalp can detect [6]. The electrical activity is measured by these electrodes, measuring scalp surface-level electrical activity. By placing electrodes of varying strengths on different areas of the brain, such as frontal, parietal, or occipital, they can pick up activities that correspond with different cycles of sleep. This cerebral activity in the brain is detected in the form of brain waves made up of tiny electrical signals the neurons give off. Thus, EEG’s are able to depict changes and variance in brain activity for people to pick up eye movement and muscle twitches during REM sleep, allowing for easier diagnosis of brain conditions.

Magnetoencephalography, or MEG (gradiometer), is a neuroimaging technique that records magnetic fields transmitted by neuron activity in the brain. This magnetic activity is measured by magnetometers and gradiometers. SQUID, a superconducting quantum interference device, is then used to amplify the neuron signals to produce tangible results. The main function of MEG is to measure brain function, something that is incredibly useful in researching what different parts, or hemispheres, of the brain are specialized in or dominant in which sleep stage [8]. By depicting where activity in the brain shows up, researchers can more accurately make hypotheses and draw conclusions about brain functions.

To work with sleep data, specifically, both EEG and MEG are very useful. Brain waves, both in the form of electrical signals and magnetic fields, allow researchers to pinpoint where and when brain activity is predominant during different parts of sleep cycles. In this case, these devices can produce data that can be organized into various temporal graphs, such as hypnograms, time series charts, etc. By studying the generated data and visualizations, neuroscientists are able to decipher brain activity in respective brain hemispheres during different stages of sleep. For instance, if the generated hypnogram depicts a notable amount of time spent in NREM-4 sleep, and if a 2D topographic brain map depicts higher intensity reds on the right hemisphere of the brain, it would support how the right brain hemisphere is dominant during the deep sleep stages, and vice versa for REM sleep and the left brain hemisphere. To determine if the individual is undergoing deep sleep, delta and theta brain waves would especially be of prominence (low frequency), while the alpha and beta brain waves would be more prominent during REM sleep (higher frequency waves)


Relationship Between Brain Hemispheric Dominance in Sleep and Nasal Cycles

In further exploring brain hemispheric dominance and its relation to the human system, its connection to autonomic nervous system cycles is especially noteworthy. Ultradian rhythms, or short-term rhythms, and circadian cycles are not only correlated with sleep stages, but with the nasal cycle as well. Airflow through the nostrils alternates mostly during the REM stage, clearly portraying the interconnection between the nasal and sleep cycles. Nasal cycles are also found to have periods of around one or more sleep cycles in length [11]. Furthermore, because of contralateral properties, where the right brain hemisphere largely is in control of the left side of the body and vice versa, the nasal cycle also embodies the same characteristics; the strengths in each hemisphere are thus evidently reflected in people's day-to-day lives. 



The understanding of sleep cycles and their functions is very important. This fundamental knowledge allows for deeper research into topics such as brain hemispheric dominance during sleep and in people’s lives. EEGs and MEGs play an especially vital role in this process, thoroughly detecting and aggregating the sleep data for researchers to analyze and visualize. Data collected by EEGs and MEGs can be visualized in the form of hypnograms or topographic brain maps, clearly illustrating the density of activity in different areas of the brain during different periods of sleep. The future of sleep research is promising and its capacity for exploration is vast.


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Sarah Dong

Sarah Dong

Hey there! My name is Sarah Dong and I'm a junior at Westview High School in California. I'm incredibly interested in neurobiology and computational neuroscience as well as analyzing neural spatial-temporal activity, and I hope that by further understanding how the brain works, we can further improve the treatment of neurological diseases. In my free time, I love to paint, bake, listen to music, and spend time with my family!