Sleep spindles are a type of brainwaves that come in bursts. They are described as an oscillatory activity of the brain which is mostly said to happen during stage 2 NREM sleep (light sleep) but it also occurs during deep sleep.
Certain relations have been found between sleep spindles and memory, circadian preferences, neural development, and unresponsiveness to outside events (ensuring tight sleep). Sleep spindle abnormality has been associated with insomnia, schizophrenia, epilepsy, mental retardation, and mental health issues.
What are sleep spindles?
Using an EEG (electroencephalogram), the scientists are able to record the activity of the brain. This activity is called brain waves. Brain waves vary depending on our state – whether we are awake or asleep. When we sleep, our brain cycles through different stages; we know this because EEG shows a distinct type of brain activity for each stage.
Sleep spindles (also called sigma bands and sigma waves) are brainwaves which burst in an oscillatory manner. They are the most active during stage 2 and deep sleep (that is, at the beginning and the end of the NREM sleep cycle).
Sleep spindles are described as slow or fast, depending on the frequency of waves. Slow spindles occur between 9Hz and 12Hz, whereas the fast spindles’ range is 12-16Hz. They do not come from the same location – slow spindles originate in the frontal brain areas, while fast spindles appear in central and peripheral parts.
Research hasn’t been able to successfully separate spindle types – in some research papers, they are only referred to as ‘sleep spindles’, without giving further details. A group of professionals from the Department of Psychiatry of Harvard Medical School, Boston, Massachusets, have pointed to this issue stating that the two types of spindles have different roles. Fast spindles are more involved in cognitive processes and memory, whereas the role of slow spindles is yet to be discovered.
K-complexes and sleep spindles
K-complexes are brainwaves which occur before and after sleep spindles. On an EEG reading, a K-complex looks like a single high-amplitude wave. K-complexes also occur as the brain’s response to outside stimuli (for example, a sound plays, something touches us, etc.). They are believed to suppress the brain arousal, maintaining sleep. Both sleep spindles and K-complexes are linked to memory consolidation and strengthening.
What do sleep spindles do? The function of spindles
The function of sleep spindles has been partially discovered, but it’s still somewhat unclear. Although scientists have worked in this field for decades, they haven’t been able to find the exact roles that sleep spindles fulfill. Here we present functions that have been scientifically proven.
Nervous system development in babies
Human and animal babies often twitch while they are sleeping – and twitching is believed to aid the sensory-motor part of the brain ‘map out’ the body, understand which exact nerve pathways correspond to which muscles.
In baby rats and human fetuses, the process is the same – a twitch followed by spindles in the matching part of the brain. In older human babies, the process becomes different but still remains similar, according to a study. It is believed to be essential for motor development and coordination.
Silencing the noise
When sounds are heard while we sleep, sleep spindles change the way the brain processes them. Research has shown that after the noise which prompted a K-complex, auditory center in the brain gets activated. The received information is distorted, making us less responsive to external noises.
People who have more sleep spindles in general also show an ability to remain asleep to outside noises while sleeping with more background noise.
Sleep spindles in light sleep (stage 2 or N2) play a role in transferring information from short-term into long-term memory, a process known as memory consolidation. It is known that during our sleep, our brain strengthens memory of how to perform physical tasks. This type of memory is called procedural memory.
In a 2016 study, a group of people was asked to learn certain motor sequences. While performing the task, an odor was released. When they went to sleep, the odor was released again in order to strengthen the memory.
Those who received odor during stage 2 sleep were the ones whose brain reacted by focusing the activity on the motor sequences learned that day. They later performed a lot better than other groups who were exposed to the odor during REM and deep sleep.
In stage 2, the number of spindles increased in response to the odor, indicating that only these spindles play a role in learning motor tasks in sleep.
A study from 2012 has shown that sleep spindles during deep, slow-wave sleep (also known as N3 stage), are generally denser than in stage 2. Researchers have also discovered that sleep spindles which appear during deep sleep also have a role in memory consolidation. To be more precise, they help consolidate declarative memory. Declarative memory is the kind of memory we employ to remember words, facts, the exact time of our meetings, for example.
This study provided important insight into the physiology of spindles during the night – the researchers have found that sleep spindle amplitude and duration do not change when we go from stage 2 (light sleep) into stage 3 (deep sleep). It is underlined that sleep spindles are present equally throughout NREM sleep. This way their findings confront a considerable amount of previous research which says that sleep spindles occur only during stage 2.
It seems that most studies focus on how our brains remember things during sleep, but we also forget in our sleep. Sleep spindles were also found to aid forgetting information which, in wakefulness, was not ‘labeled’ to be remembered.
We are able to discriminate between important and unimportant information. Nerve pathways determined for remembering are strengthened, while others are ‘cleared out’ from our memory if needed.
Some researchers insist that when searching for the sleep spindle functions, one should pay close attention to whether the subjects are male or female because it seems that there is a significant difference between the two genders.
Some studies suggested there was a correlation between fast sleep spindles and intelligence quotient (IQ) score. However, it seems that only in women the IQ is related to the number of spindles – in men, no such correlation has been found.
When testing women only, a study shows that memory fluctuates with their menstrual cycle, as a consequence of estrogen influence. As declarative memory is closely tied to sleep spindles, it suggests that spindles in women vary depending on the part of the cycle. This is why they advise against putting women and men in the same research category as it may produce false results.
Sleep spindles of our chronotype
It seems that our chronotype (daily rhythm of our own preferences for the sleep and wake time) is in relation to the sleep spindles our brain produces during sleep. Sleep spindles in morning types have significantly ‘weaker’ spindles, that is, they are of lower intensity and amplitude compared to afternoon and evening types.
Spindles of the morning types were also reported to have a fast decrease towards morning. Other two groups did not have such a change.
Sleep spindles and insomnia
Those suffering from insomnia have a higher level of brain activity (cortical hyperarousal) – their sleep spindles are either denser or just different from people who don’t have these problems. A study conducted among students had the aim of finding a correlation between academic stress and sleep spindles. They found that low spindle activity at the beginning of the night successfully predicted sleep insomnia-related problems caused by stress.
It seems that sleep spindles in various other sleep disorders, like those connected with excessive daytime sleepiness, like narcolepsy and hypersomnias. Some research has found a connection between abnormal sleep spindles and mental retardation in children.
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