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The activity of our brains depend on the state that we are in, and it can be measured with an electroencephalogram (EEG). The neurons in our brain have to communicate all the time, and they can do that via electrical and chemical signals. Chemical signals are in the form of different neurotransmitters and neurohormones that have many functions. Electrical pulses happen because of the influx and efflux of ions through the cell membrane in neurons. It is a faster way to transmit the information, and it’s also better suited for long distance signaling.
The oscillatory neural impulses in the central nervous system are called brain waves. There are four different basic types of brain waves. Beta waves happen when we are awake, active, and engaging in some activity that includes cognitive involvement. Alpha waves represent the non-aroused resting state. Theta waves occur during light stages of sleep, while delta waves have the lowest frequency, and they are reserved for deep, restorative sleep.
Sleep spindles and K-complexes are electromagnetic waves that are indicators of non-REM sleep.
What Are Sleep Spindles
There are mainly two types of sleep, rapid eye movement (REM) and non-REM sleep (NREM). NREM sleep is composed of three stages. First two are attributed as light sleep, while the third one is considered a phase of deep rest. Each sleep cycle has all of the stages, and it usually lasts around 90 minutes.
Stage 2 of non-REM sleep is where we spend most of our time while sleeping, as much as 50% of total sleep time. It is a time when our breathing and heart rate slow down, your senses are lowered, and your brain is producing theta waves with an amplitude of 4 to 8 Hertz.
Sleep spindles are exclusive for Stage 2 sleep. They are oscillatory neural activity generated in the thalamus during this part of light sleep. They are called spindles because of how they look when the EEG image is printed out. Sleep spindles happen in short bursts that last from 0.5 to 1.5 seconds and register frequencies of 11 to 15 Hertz. There are essentially two types:
- Slow spindles, with frequencies from 11 to 13 Hertz, are produced in the frontal cortical brain regions.
- Fast spindles that record frequencies from 13 to 15 Hertz, and are usually produced in central and parietal brain sections.
Sleep spindles begin to develop when the baby reaches six weeks of age. Research indicates that some essential brain activities occur while sleep spindles are happening, whether it is during nighttime sleep or daytime naps.
What Are K-Complexes?
K-complexes (KCs) appear only in Stage 2 of NREM sleep, just like sleep spindles. Unlike short bursts of activity, KCs are large waves that stick out on an EEG reading.
KCs form as a reaction to some stimulus in the environment, including noise, touch on the skin, or any other external stimuli. They are usually accompanied by sleep spindles, as your brain tries to stay asleep, and not wake up due to external distractions.
K-complexes are found to develop much later than sleep spindles, and they can be observed in six-month-old babies.
Abnormal K-complex activity has been with some sleep disorders, including restless legs syndrome (RLS), obstructive sleep apnea, and also epilepsy. For instance, in RLS, there is an increased number of KCs just before the leg movements.
The Importance of Sleep Spindles
Since we spend so much time in Stage 2 sleep, there must be something pretty important happening during that time. Scientists are still trying to figure out everything about them, but here are some things that we do know.
One of the primary sources of sleep spindles is thalamus, a part of the brain that is in charge of processing outside sensations, and it also plays a role in regulating sleep. Other important sources are cortical regions that are in charge of encoding memory. Sleep spindles are thought to be an indicator of the transfer of information from the hippocampus which is considered to be fast short-term storage, to neocortex that is a supposed to be a place for storing long-term memories.
That is why sleep spindles are correlated with better memory and higher index of intelligence. For instance, when a person is learning something new, the frequency of sleep spindles increase during the following night. It also goes both ways, the more spindles a person produces during the night, the better he performs the skill that he learned the previous day. That’s why scientists firmly believe that spindles are a sign a memory transfer.
Stage 2 of sleep is also found to last longer in the second part of the night, so cutting your nightly slumber short may not be the best idea. That is why you feel that cognitive decline after a day of sleep deprivation. You haven’t experienced as much sleep spindles, and your concentration, memory, and ability to learn are impaired.
And this is consistent with EEG findings in babies and the elderly. There are more sleep spindles recorded in babies, which makes a lot of sense since they are developing and learning, so their brains are like a sponge, just soaking in all the information from the outside.
When it comes to older adults, the number of sleep spindles during resting period decrease. That is correlated with declining of cognitive and motor skills, as well as disrupted sleep.
Sleep spindles play another vital role in protecting the integrity of our sleep. It is shown that higher spindle density means that a higher amount of noise is needed to wake the person up, so people who produce more spindles get a night of better quality sleep. This makes sense as thalamus is the primary source of spindles, and it also plays a role in receiving information from the outside, so they are kind of like a gatekeeper.
We already mentioned that older people experience fewer sleep spindles, and they also have more disrupted sleep, so they are easily woken up during light sleep, and it is harder for them to accomplish deep sleep, that is needed to restore correctly.
Fortunately, melatonin supplements seem to promote sleep spindles, so they have been a helpful aid for people that can’t seem to get sufficient sleep.
Sleep Spindles and Medical Conditions
Because of their connection to sleep, memory, and cognitive functions, sleep spindles are studied as a possible signal of many brain dysfunctions. Different conditions such as Parkinson’s disease and Alzheimer’s have sleep impairment and cognitive decline as usual symptoms. It is possible to track sleep spindles easily through EEG, and that’s why scientists are using them to monitor how functions of the brain change.
A study from 2012, found sleep spindle abnormalities in patients diagnosed with schizophrenia. Sleep disruptions have long been a known symptom of this disease, as well as reduced memory consolidation. The patients in this study are observed to have fewer sleep spindles than people who don’t suffer from this disease, and it can be an explanation for the symptoms they are experiencing, considering the roles of spindles that we’ve discussed.
Studies done on people who have Parkinson’s and Alzheimer’s disease show similar results. They also experienced loss of sleep spindles, which was correlated to worse sleep quality and impaired cognitive ability. Sleep spindles in patients with Parkinson’s disease are also found to last longer, oscillation frequency was slower, and maximum amplitude was higher, compared to healthy people in the control group in the study.
People with MDD or major depressive disorder have a denser spindle activity, which implies that there is a connection between memory regulation and brain’s ability to process memories.
There is still a lot of research to be done in this field of sleep, memory formation, and how it is all connected. What we know so far is that sleep spindles play an important role in learning, sleeping, and they could be a good indicator of intelligence. They can be easily tracked with EEG, so there is excellent potential in observing sleep spindles and predicting a chance of developing some of the neurodegenerative diseases.