Stages of Sleep

It may be easy to simply quantify the act of sleeping as one extended period where the body can obtain rest. However, the act of sleep is far more complex what this blanket statement makes it out to be. And one of the biggest ways to demonstrate the level of complexity that can be linked to the stage of sleep has to do with the pattern of sleep itself.

Rather than sleep being viewed as one continual interval of time in which a body can rejuvenate, its overall process should be viewed as a gradual process. The reason for this is because the actual act of sleep itself is a cycle of natural activities that occurs within the brain. This cycle is essentially marked by different stages of progressive behavior, which repeat themselves once the cycle is complete, and ultimately once the person enters a state of wakefulness.

A Look at the Sleep Cycle

In essence, the sleep cycle is divided into two separate components:

  • REM, or Rapid Eye Movement, Sleep
  • Non-REM Sleep

In total, there are four stages that make up the sleep cycle, although some studies split one of the stages to form a five stage cycle. The cycle consists of three stages of Non-REM sleep as well as one stage of REM sleep before the entire process starts anew.

The first stage, NREM stage 1, is representative of the stage that exists between sleep and wakefulness, and it is typically shown to last anywhere from five to 10 minutes. There is still ambulatory and sensory awareness at this particular stage, although the eyes are closed. Because this is at the beginning phase of the cycle, it can be relatively easy to awaken a person in this stage without too much of an effort. A person that is awoken at this particular phase may also feel as if they did not fall asleep. Additionally, it is not uncommon for a person to experience hypnic myoclonia; a condition that is marked by a sensation of falling which could in turn cause a sudden contraction of a muscle.

The second stage, NREM stage 2, is a phase where the body starts to prepare itself to enter into a state of deep sleep. During this particular stage, the heart rate will slow down and the body temperature will drop. The muscles will also show exhibit period of muscular relaxation mixed with spontaneous periods of muscle contraction. At this stage, it becomes more difficult to awaken a person from sleep.

The third stage, NREM stage 3, is where deep sleep occurs. This stage is also known as delta sleep or slow-wave sleep, or SWS for short. This is the segment of the cycle where it has been determined that a body's anabolic processes reach their apex. These processes relate to several systematic functions as they relate to protein synthesis. Some of these functions include the muscle and tissue repair, the regeneration and proliferation of muscular and skeletal tissue, an increase of bone density, and a strengthening of the immune system. It becomes very difficult to arouse someone from sleep if they are locked into this particular stage, as many environmental stimuli that would typically evoke some sort of reaction would not produce the typical response. If they are aroused, they may feel disoriented for a few minutes. It should be noted that the time interval of this particular stage diminishes as part of the aging process.

A Look at REM

The final stage of the sleep cycle, REM or rapid eye movement, is the most complex of all the cycles. Typically, the REM stage is entered at roughly 90 minutes after a person fall asleep. In this particular stage, the voluntary muscles are paralyzed. Yet at the same time, this stage is marked by brain activity that is comparable to the activity that has been marked during the brain's waking stage. Furthermore, vital signs such as arousal and the consumption of oxygen by the brain occur at a rate that is higher than they do when a person is awake. Because of this, REM is sometimes referred to as paradoxical sleep. REM is also where the vast majority of dreaming takes place, although it has been noted that less vivid dream can occur in other stages of sleep. While the actual functionality of this stage is uncertain, it has been determined through scientific study that its activity supports daytime performance. It has also been determined that the prevention of its occurrence could impede the ability to learn complex tasks.

The Cyclical Process

Typically, the stages of sleep repeat themselves about four to five times each evening within the parameters of the recommended sleep range of an adult. It has been determined that the process itself is not a perfectly gradual cycle, as NREM stage 2 will re-appear between the end of NREM stage 3 and REM. The length of the stages within the cycle is also not rigid, as their length is essentially determined by where a person is at in their overarching sleep interval. For example, there is a greater amount of NREM stage 3 sleep - and therefore, deep sleep - earlier in the sleep interval, whereas the REM stage lengthens in the two cycles that occur before the person enters a period of wakefulness. The fact that the length of REM extends with each rotating cycle, coupled with the brain-related activity that occurs during the stage, at least partially explains the reason as to why a good night's sleep is critical for a person's body and well-being.

Throughout the entire sleep interval, it is been shown that the NREM stages of sleep do combine to comprise 75 to 80% of the overarching interval, with REM making up the remaining 20 to 25%.

How the Stages of Sleep are Measured

The stages of sleep and the resultant brain activity that occurs during the various stages are determined via the measurement of brain waves. The most common way that sleep stages are broken down and scrutinized is through a process called polysomnography. This is a multi-parametric test that essentially puts forth a comprehensive recording of the various biophysiological changes that have been shown to occur during the process of sleep. The information that is gathered is done so by way of three devices. The first of these devices is an electroencephalogram (EEG), which is used to provide a summary of electrical activity from a particular region of the brain. The second device is an electromyogram (EMG), which is used to measure muscle tone. The third device is an electro-oculogram (EOG), which is used to study eye movement. The test result that is derived from these machines is known as a polysomnogram, or PSG. There are several bodily functions that a PSG will measure during sleep in conjunction with these various studies, such as functions associated with the brain, eye movements, muscle activity or the activation of skeletal muscle, heart rhythm, and respiratory airflow. The chief reason that polysomnography is used to scrutinize the various stages of the sleep cycle is to diagnose certain sleep disorders such as REM behavior disorder, sleep apnea, parasomnias, and narcolepsy.

There have been various findings that have been culled from these particular charts. For instance, it has been determined that the percentage of each sleep stage does vary by age. The majority of sleep that is done by all ages except infants is performed at NREM stage 2. Furthermore, it has been shown that intervals of deeps sleep and REM decrease as a person ages. It has also been determined that other outside influences can have a notable impact on the sleep cycle, most notably sleep deprivation, alcohol taken before bedtime, and drugs, in particular pain medications and anti-depressents.

Stages of Sleep and Sleeping Disorders

The use of polysomnography to detect certain sleeping disorders has resulted in linking certain issues related to sleep to the various stages in which they occur. For example, it has been determined that some people may experience a phenomenon known as hypnagogic hallucinations during NREM stage 1. In other words, a person in this stage may experience a sensory malfunction as a result of transitioning from wakefulness into the early stages of sleep. It has also been determined that NREM stage 3 is the stage where a host of parasomnias manifest. This would include night terrors, nocturnal enuresis, somniloquy, and sleepwalking.

More to Discover

While there has been a lot of advances relating to the study of sleep intervals and the stages of sleep that collectively constitute the sleep cycle, there is still plenty of elements that pertain to sleep and its various stages that remain unknown. However, as more research and study is devoted to this aspect of sleep - not to mention sleep in general - it becomes clear that the answers that remain elusive in the here and now will be attainable one day through a blend of technological advance and scientific curiosity. It is only a matter of time until the unknown is revealed.