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Analysis of electroencephalographic (EEG) rhythms helps uncover a little of the mystery of what goes on while we sleep. If you place electrodes on the scalp of a sleeping person, you can see patterns of rhythmic discharges of neurons through the thalamocortical system. We experience different types of brainwaves as we progress through the different stages of sleep. Sleep spindles and K complexes represent two of these electromagnetic waves.
Sleep spindles are sudden bursts of oscillatory brain activity generated in the reticular nucleus of the thalamus that occur during stage 2 of light sleep. These brainwaves are called sleep spindles because of how they look when printed out on an EEG reading. The EEG shows voltage difference fluctuations, and the lumping together in a close time of a flurry of fluctuations suggests something going on in the brain like a transfer of electrical energy.
Sleep spindles may also be referred to sigma bands or sigma waves. The electrical activity that gives rise to spindles is both global (over much of the brain) and local (only in parts of the brain). In the brain the thalamic and corticothalamic networks are involved with the electrical activity that produce a spindle on an EEG. Physiologists are trying to distinguish among different types of spindles. Fast spindles (13–15 Hz) occur in the centroparietal part of the brain, while the frontal brain produces slow (11–13 Hz). Increased spindle activity occurs as the onset and outset of light sleep.
Sleep spindles begin to develop once an infant has reached six weeks of age, and may explain why babies twitch in their sleep. EEG typically will display sleep spindles immediately after muscle twitching.
Researchers believe sleep spindles represent periods of time where the brain inhibits mental processing in order to keep the person in a tranquil state. By keeping the person in a tranquil state, the sleep cycle can continue and the person can transition to the next stage of deep sleep.
Like sleep spindles, K-complexes are defining brainwaves of stage 2 sleep. They differ from sleep spindles in their form. Unlike the rapid burst of activity represented by sleep spindles, K complexes are large waves that react to external stimuli while sleeping.
K-complexes also develop later than sleep spindles. Sleep spindles develop at about 6 weeks of age, while K-complexes do not show up until about 5 months.
K-complexes form as a reaction to external stimuli in the bedroom or outside while a person is asleep. Sleep spindles follow K-complexes as the brain works to stay asleep.
Abnormal K-complex activity is linked with epilepsy, restless legs syndrome (RLS), and obstructive sleep apnea. For example, RLS sufferers experience a higher volume of K-complexes, which typically happen right before the leg movements. This increased K-complex activity may contribute to the less restful sleep associated with restless legs syndrome.
We spend almost half of the night in stage 2 of sleep (45%). During this stage our body temperature drops and our heart rate slows as we prepare for restorative deep sleep. Light sleep is so-called light because it is easier to wake a person up from sleep, which is why K-complex waves occur during this stage as a response to external stimuli.
Research has shown that sleepers who produce spindles more frequently tend to require a higher amount of noise to be woken up. This indicates that people with higher levels of spindle activity are likelier to enjoy higher-quality sleep.
This theory also aligns with the changes in sleep architecture that people experience as they age. As you get older, you produce fewer spindles, so your sleep is more likely to be interrupted before you reach deep sleep. Because the brain isn’t producing the required amount of sleep spindles to maintain a restful state, seniors experience lower quality sleep.
Fortunately, melatonin seems to promote spindles, which may explain why many people find it beneficial as a sleep aid.
Spindles indicate a transfer of information between the hippocampus and the neocortex. The frequency of spindles seems to indicate how active the brain is and serves as a physiological index of intelligence.
For example, sleep spindle frequency increases during the night following when a person has learned something new. Both the density (number in a given period of time) and sigma (14-16Hz) spectral power go up after a nap as performance and vigilance measures go up.
Furthermore, when there are more spindles, the person’s performance at a recently learned task or skill increases the next day. This is why scientists are convinced spindles indicate a transfer of memory from short term (daily) to long term memory. How memory is stored in the brain is still a great mystery, but at least we can see indication of long-term memories forming on an EEG.
Sleep spindles are more prevalent in children who are maturing and learning, further strengthening the association between spindles and intelligence. These spindles may represent the brain processing and integrating new information during light sleep. This is why when you experience sleep deprivation, your memory and cognitive abilities to suffer – your brain is not processing the same amount of sleep spindles as it should.
Spindles may be used as a biomarker for schizophrenia some day, since people with schizophrenia display an abnormal pattern of fast and slow spindles, as well as reduced levels of sleep spindles overall, when compared to the average population.