Introduction
Studying the ways people sleep, their patterns of sleep, and discovering the type of sleep known as REM sleep, all were aided by the invention of the electroencephalograph (Cherry, n.d.). Discoveries made included that when humans sleep, there is a progression through a number of stages, during which the subject’s patterns of brain waves change. In this essay those stages and their sequence are described, including that special type of sleep called REM, or Rapid Eye Movement sleep, which contrasts with the other stages of sleep, sometimes accordingly referred to as NREM, or Non-Rapid Eye Movement sleep.
The Stages of Sleep
At the beginning of sleep, Cherry described how the subject is comparatively alert and almost awake. In this preliminary stage the brain’s neural networks generate small and rapid brainwaves called beta waves. As relaxation increases, the brain processes become slower, and alpha waves are generated. During this “almost asleep” phase, subjects may experience a feeling of falling or of someone calling them; these odd but quite vivid feelings are known as “hypnagogic hallucinations.” Another fairly common happening in that same early sleep state is to experience sudden involuntary muscle spasms known as “myoclonic jerks.”
Cherry’s article then described the five stages of actual sleep, but noted that the five stages do not totally follow in the progression described (see later). Stage 1 is the first – and lightest – stage of actual sleep. It can be thought of as a transitional stage between being awake and fully asleep. During this relatively short-lasting stage (perhaps between five and ten minutes), the brain waves produced are very slow but have high amplitude and are known as theta waves. Cherry stated that someone woken while still in that stage of sleep might well think they were not really sleeping. Then stage 2 sleep begins and lasts for 20 minutes or so, during which time the brain generates bursts of rapid and rhythmical brain waves called “sleep spindles.” During stage 2 sleep the heart rate slows and body temperature starts to fall.
Following stage 2, stage 3 sleep – a transition between the so far light types of sleep and a much sounder sleep – is epitomised by the start of much deeper and slower brain waves called delta waves. Those continue into the stage 4 sleep – sometimes called “delta sleep” as a consequence, which lasts usually for circa 30 minutes. Cherry noted that it is often at the end of the stage 4 sleep phase that sleepwalking or wetting the bed are most likely to occur.
Then there is the stage 5 type of sleep, which is REM sleep (see separate section following). However, as Cherry noted, the five stages described do not follow in that sequence. Sleep stages one to four occur sequentially, but then – after stage four – stage three repeats, followed by stage two, then the stage five (REM) sleep occurs. Following that, the sleeping person usually returns to stage two sleep again. This complete cycle is normally repeated perhaps four or five times over the duration of a night.
REM Sleep
According to Cherry, Eugene Aserinsky used the electroencephalograph in the mid-twentieth century to discover the phenomenon of REM sleep, which is characterized by much eye movement, and by increases in both brain activity and the rate of respiration. The Cherry article noted that paradoxically whilst the activity of a person’s brain and other systems increases during REM sleep, the subject’s muscles are more relaxed. Taking a typical pattern of sleep throughout a night, the first REM stage may be after about 90 minutes, and of only short duration. However, the duration of each successive REM cycle increases, and can eventually last for as much as an hour. Most dreams are experienced during REM sleep.
According to the Carskadon and Dement article (2011), the sleep cycles as described are age-related. They claimed that newborn babies experience REM sleep before NREM sleep and have a shorter overall sleep cycle. The pattern and the proportion of REM sleep then changes as they grow, falling from about 50 percent REM at birth to perhaps between 20 and 25 percent at two years of age. The authors also stated that stages three and four sleep types durations decline progressively into old age, especially in men.
Regarding the purpose and function of the various sleep states, including REM sleep, there is little solid information. Purves, Augustine, Fitzpatrick, et al. (Eds.) (2001), discussed these matters in The Possible Functions of REM Sleep and Dreaming. The authors noted that although the body of opinion in sleep research is that the non-REM sleep states are in part necessary for the body to recharge its batteries, the purpose of REM sleep has not been ascertained with any degree of certainty. Although it is well known that dreams occur mostly during REM sleep, it is unclear whether dreams in themselves serve any purpose. Some have suggested they have an “unlearning” function, erasing from the brain’s neural networks thoughts that could otherwise develop into unwanted mental activities such as paranoia. Another view is that the dreaming process reinforces behaviours not encountered while awake, by effectively “rehearsing” them.
Conclusions
It has been found that the human sleep cycle follows a definite pattern, moving through stages of identifiably different sleep states, including Non-REM and REM stages, repeating through the duration of a normal night’s sleep. It was also found that those patterns vary with age, changing especially rapidly in the two years or so following birth, but continuing to evolve into old age. Whilst Non-REM sleep is thought to be contributory to the restoration of a person’s energy, the purpose of REM sleep has not been ascertained, though various theories exist.
Works Cited:
Cherry, Kendra. (n.d.). Theories of Sleep. About.com. Web. 15 February 2013.
Carskadon, M., A., & Dement, W., C. (2011). Normal Human Sleep: An Overview. Web. 15 February 2013.
Purves D., Augustine G., J., Fitzpatrick D., et al.(Eds.), Sunderland (MA). (2001). The Possible Functions of REM Sleep and Dreaming. Web. 15 February 2013.
