From a medical point of view, sleep can be understood as a state of mind experiencing reduced levels of consciousness involving a temporary inactivity of nearly all voluntary muscles, and a relative suspension of sensory and non-motor activity. In simple terms, sleep is an impermanent physical and mental state of the mind during which most of the external stimuli are blocked from the senses, and the individual stops responding to the environment. There are different types of "sleeps" depending upon the intensity and manifestation of sleeping criteria. The ability of the person to "awake", or come out from the transitory partial inactive state of the mind depends upon several factors, and these factors vary from person-to-person. Even though it can't be proved on a conclusive basis, medical experts believe the basic purpose of sleep is to create a state of inertness in the human body, during which the body can repair itself and regulate the metabolism to improve its functioning, and the state of inertness helps save the energy which is utilized for the rejuvenation process.
Repair theory
As per this theory, during the "awake" period, the body is physically and mentally responding to the various activities associated with our daily activities and this consumes large amounts of energy. The energy utilized is depleted from the energy reserves stored in various parts of the body. The replenishment and body repair activities can occur effectively when the body is undergoing a state of rest - when no extra energy is utilized for any physical processes or activities. The sleep period ensures all the resources in the body are utilized in an optimum manner for the maintenance and up-keep of the various metabolic processes occurring in our body that keep us alive.
Adaptive theory
According to this theory, sleep is a naturally evolved phenomenon that humans and animals adapted to for their survival. Sleeping helps in preserving energy, and prevents exposure to dangers and predators.
How "sleep" works?
As far as the process of sleeping is concerned, scientists believe that our metabolism has two processes:
? The sleep-awake process
? Circadian biological clock or Circadian rhythm
which regulate our sleep. Both the processes function in tandem, and create the "sleep cycle" because of which we tend to feel sleepy at night, and remain awake during the day. The processes regulate our sleep cycle, which scientists believe is essential for body repair and sustenance. The Circadian biological clock can be understood as a 24-hour condition during which the body rhythm is affected by sun light. The presence and absence of sunlight controls the secretion of certain essential hormones in the body. The melatonin hormone (N-acetyl-5-methoxytryptamine) is secreted in the absence of sunlight, generally at night, and is primarily responsible for regulating the body temperature. The cycle needs to be in accord with the physical state of the individual and the metabolic functioning of the body.
Circadian rhythm or the "sleep cycle"
A Circadian rhythm is an approximately 24-hour cycle associated with the biochemical, physiological, and/or behavioral processes. The term "circadian" is derived from the Latin word "circa" which means "around", and "diem" or "dies" which means "day". Therefore, the term literally means "approximately one day". The rhythm is generated by a metabolic activity which functions as an internal body clock, and which is synchronized with the "light-dark" cycles as well as the changes taking place in the subject's environment.
Stages of sleep
The sleep process consists of two main stages which keep on repeating in a cycle of 90 to 110 minutes during the entire sleeping activity. The two stages are:
? REM (Rapid Eye Movement)
? Non-REM (Non - Rapid Eye Movement, which is further classified into four sub stages)
REM sleep
The Rapid Eye Movement (REM) stage of sleep is characterized by a rapid movement of the eyes, in addition to a low muscle tone, and/or a partial paralysis of all voluntary muscles. In case of humans, this type of sleep occupies between 20% to 25% of the total sleep duration - approximately 90 to 120 minutes. Typically, about four to five cycles occur during normal REM sleep. The duration of the cycle is short in the beginning of the sleep, and starts extending towards the end. The exact duration of REM sleep required can't be ascertained, since the sleep cycle varies from individual to individual as per the body's metabolic requirements. In case of newborn babies, the REM stage consists of around 80% of the total sleeping time. REM sleep is also affected by the aging process. During the REM stage, no dominate brain waves are emitted as per polysomnogram findings. The first REM cycle usually begins approximately 70 to 90 minutes after the sleeping process commences i.e. after falling asleep. During the REM stage, even though the subject is not responsive to any strong external stimuli, the brain remains active, and as per polysomnogram tests the degree of activity is considerably more as compared to the "awake" stage. The REM stage is also associated with the "dreaming" phenomenon, and when the process occurs, the frequency of REM i.e. the movement of eyes increases significantly. Typically, depending upon the intensity of the dreaming activity, the blood pressure too is affected, and can increase marginally or significantly.
Non-REM sleep
The Non-REM sleep stage is characterized by an absence of rapid eye movement, a decrease in the metabolic activity, a reduction in the breathing and heart rates, and generally a substantial decrease amounting to almost an absence of dreaming activity. Unlike the REM stage, voluntary muscles do not experience a partial paralysis in this stage. Non-REM stage is composed of four sub stages - stage 1 associated with light sleep, stage 2 by true sleep, and stages 3 and 4 with deep sleep. In human adults, the Non-REM sleep comprises about 75% to 80% of the total sleeping time. The stages are as follows:
Stage 1: Light sleep
During the "awake" stage, the brain emits alpha waves having a frequency between 8 to 13 Hz. On the onset of the first stage of Non-REM sleep, the brain undergoes a gradual transition during which the intensity of the waves emitted starts decreasing, and reaches between 4 to 7 Hz characterized by the theta waves. This stage may involve slow eye movement, twitches, and even "hypnic jerks" commonly referred to as "sleep start" or "night start" during which the subject awakens suddenly just when he or she is about to fall asleep. The voluntary muscles and metabolic activities start slowing down. The individual can be easily awakened during this stage of sleep.
Stage 2: True sleep
Generally within 10 to 15 minutes of the first stage of light sleep, the second stage of true sleep sets in which lasts approximately between 20 to 25 minutes. This second stage of Non-REM sleep is characterized by "sleep spindles" ranging from 11 to 16 Hz during which the brain inhibits various processes to keep the subject in a tranquil state, and the K-complex which suppresses cortical arousal to prevent any external stimuli from signaling danger and aid sleep based memory consolidation. During this stage no eye movements occur, and the breathing pattern as well as the heart rate slows down. This stage comprises about 45% to 55% of the total sleep consumed by adults.
Stages 3 and 4: Deep sleep
During the third stage of Non-REM sleep, the brain starts emitting delta waves having high amplitude (75 µV) and low-frequency (0.5 to 2 Hz). The breathing and heart rates are at their lowest. Parasomnias - a category of sleep disorders associated with abnormal and unnatural body movements, abnormal behavior pattern, uncontrolled emotions, and abnormal perceptions generally occur during this stage. Sleeping disorders such as night terrors, sleep walking, nocturnal enuresis (bedwetting), and somniloquy (talking aloud in one's sleep) are also associated with this stage. The fourth stage of Non-REM sleep is associated with rhythmic breathing and restricted muscle activity.
Dreams
From a scientific point of view, there is no fixed definition of "dreaming". A dream can be interpreted as a succession of images, sounds, and emotions that the human mind experiences during the sleeping process. The basic purpose and manifestation of the dreaming phenomenon is still not clearly understood, and scientists have several hypotheses which try to explain the process. However, it is commonly believed amongst scientists that dreams are basically a result of certain psychological, or neuropsychological activities occurring in certain portion of the brain. Dreams are associated with the psychological aspect of the brain - the non-tangible part (mind), and not the tangible part (brain). It's important to differentiate between the two. During the lifetime, it is believed a human being spends approximately six years dreaming, which comes to around two hours on a daily basis as an average, if we are to consider the average life span of a person. It is still unknown exactly how and why dreams originate, and whether there is a single origin, or multiple portions of the brain are involved.
Difference between REM and Non-REM sleep dreams
There is a subtle difference between the dreams occurring during the REM and Non-REM stages of sleep. Dreams occurring during the Non-REM stage are brief and fragmentary, do not have any lasting impression on the individual, and they are easily forgotten. In this particular stage, the individual is less likely to experience any lucid and clear visual images resulting out of the dreaming process. During the REM dream stage, a portion of the brain called "pons" shuts off all chemical signals associated with voluntary muscles functioning to the spinal cord. This causes temporary paralysis, and the body becomes incapable of any voluntary movements. The REM sleep signal originates from the pons. This is a natural defense mechanism which prevents the person from harming himself or herself during sleep, since the dreams occurring during the REM stage of sleep can seem to be real and life-like, and this can cause the subject to respond physically in accordance to the particular dream pattern. It is believed pons secretes acetylcholine, a chemical compound that acts as a neurotransmitter, during the REM stage which is transmitted to different parts of the forebrain. This causes cholinergic activation within the affected tissue areas. This is what causes the "dream" phenomenon.
Mandar Majmudar is a professional content writer who has worked extensively in the BPO and KPO segments since last many years. He has undertaken many site projects involving the designing and content writing aspects. He has trained professional content and technical writers for MNCs, and small to medium business entrepreneurships. He specializes in creating concept based web contents - the client provides a concept, and he works out the different types of content formats to promote the concept on the internet. He contributes articles to various sites and forums.
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