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Usually sleepers pass through four (4) stages of sleep:
These stages progress cyclically from 1 through to REM, then begin again with stage 1.
A complete sleep cycle takes an average of 90 to 110 minutes, with each stage lasting between 5 to 15 minutes. The first sleep cycle each night has relatively short REM sleeps and long periods of deep sleep but later in the night, REM periods lengthen and deep sleep time decreases.
So, let’s take a closer look at each stage of the cycle and explain what scientists think happens throughout these stages.
STAGE 1 is the lightest stage of Non-REM (NREM) sleep. Often defined by the presence of slow rolling eye movements, this drowsy sleep stage can be easily disrupted causing awakenings or arousals. Muscle tone throughout the body relaxes and brain wave activity begins to slow from that of wake. Occasionally people may experience jerks or abrupt muscle spasms and may even experience sensation of falling while drifting in and out of Stage 1.
STAGE 2 is the first actual stage of defined NREM sleep. Awakenings or arousals do not occur as easily and the slow-moving eye rolls discontinue. Brain waves continue to slow with specific bursts of rapid activity known as sleep spindles intermixed with sleep structures known as K complexes. Both sleep spindles and K complexes are thought to serve as protection for the brain from awakening from sleep.
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. 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 more likely to enjoy higher-quality sleep.
which is a small organ located within the brain's medial temporal lobe and forms an important part of the limbic system, the region that regulates emotions. The hippocampus is associated mainly with memory, in particular long-term memory. The organ also plays an important role in spatial navigation
Spindles indicate a transfer of information between the hippocampus (which is a small organ located within the brain's medial temporal lobe and forms an important part of the limbic system, the region that regulates emotions. The hippocampus is associated mainly with memory, in particular long-term memory) and the neocortex ( a part of the cerebral cortex along with the archicortex and paleocortex, which are cortical parts of the limbic system). It is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought, and in humans, language. The frequency of spindles seems to indicate how active the brain is and serves as a physiological index of intelligence; the more spindles the greater 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) 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. Therefore, 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. Therefore, when you experience sleep deprivation, your memory and cognitive abilities suffer.
Spindles may be used as a biomarker for schizophrenia, 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.
STAGE 3 is known as DEEP NREM sleep. The most restorative stage of sleep, stage 3 consists of delta waves or slow waves. Awakenings or arousals are rare and often it is difficult to awaken someone in Stage 3 DEEP NREM sleep. Parasomnias (sleepwalking, sleep talking and night terrors) occur during the deepest stage of sleep.
Deep sleep reduces your sleep debt and provides the most restorative sleep of all the sleep stages. This is why if you take a short nap during the day, you’re still able to fall asleep at night. But if you take a nap long enough to fall into deep sleep, you have more difficulty falling asleep at night because you reduced your sleep debt or need for sleep.
During deep sleep, human growth hormone is released and restores your body and muscles from the stresses of the day. This is an important part of maintaining mental and physical performance. In fact, this process is somewhat antiaging as your DNA is repaired during DEEP NREM sleep.
Your immune system also restores itself. It is during this stage that the brain also refreshes and cleans itself for new learning the following day. This includes the removal of beta amyloid plaque and excess TAU protein. These proteins and their build up have been associated with Alzheimer’s disease, dementia and memory loss.
REM sleep, also known as rapid eye movement, is most commonly known as the dreaming stage. Eye movements are rapid, moving from side to side and brain waves are more active than in Stages 2 & 3 of sleep. Awakenings and arousals can occur more easily in REM; being woken during a REM period can leave one feeling groggy or overly sleepy.
Longer REM stage sleep occurs in the later part of the sleep period instead of the longer deep NREM sleep earlier in the sleep.
A person typically experiences three to five REM periods throughout sleep time with the longest REM period right before awakening for the day. If woken prematurely from an alarm clock or other reason a person can experience a period of sleep inertia whereby a heightened sensation of sleepiness or drowsiness can occur for several minutes or even several hours. This is why it is a good idea to plan your sleep so that you are going to bed 8 hours before you wish to wake.
In the REM period, breathing becomes more rapid, irregular and shallow, eyes jerk rapidly, and limb muscles are temporarily paralysed. This muscle paralysis occurs as a protective means to keep a person from acting out their dreams.
Brain waves during REM stage increase to levels experienced when a person is awake. Also, heart rate increases, blood pressure rises, males develop erections and the body loses some of the ability to regulate its temperature. This is why its often the last stage of sleep before waking and the longest period of REM is directly before you wake.
As you can see all stages of sleep are important and healthy sleep hygiene will support a balanced circadian (wake sleep cycle) rhythm. If you get to bed too late you will miss good quality DEEP NREM sleep and if you get up too early you may miss critical REM sleep.
We have combed through the research and looked at our evolutionary past for how to plan the perfect sleep. We share this information in a blog called “THE TRUE COST OF INSOMNIA”. Make sure you download today!
Written by Greg Haglund
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