All of us love sleep. After a meal we feel sleepy, when we do a lot of work we feel tired and require rest in the form of sleep. Sleep refreshes our mind and provides us better power to concentrate on our tasks. Truly speaking sleep is a condition which is coordinated by both the sensory and motor nerves recognized by the state of complete or partial unconsciousness where all the voluntary muscles are inactive. It is a state that is different from hibernation or coma. It has been found that all mammals, birds, certain reptiles, amphibians and fishes also sleep. Studies show that in all animals sleep plays an important role in survival. The mechanisms under sleep are still the topic of scientific research.
In all animals sleep is divided into two broad types. First one is the rapid eye movement (REM) and the second one is the non-rapid eye movement (NREM). Both these phases have different physiological, neurological and psychological characteristics. The American Academy of Sleep Medicine has further divided NREM sleep into three stages which includes the NI, N2 and the N3 stages. N3 stage is also termed as the delta or the slow wave sleep (SWS). The sleep cycle starts with the REM stage which is preceded by the NREM stage. The early part of the night constitutes of the deep sleep and the later part of the night includes the NREM sleep followed by the natural awakening. Alfred Lee Loomis and his co-workers in 1937 described the stages of sleep from A to E which includes the stages from wakefulness to the deep sleep for the first time. William Dement and Nathaniel Kleitman in 1953 reclassified the stages of sleep as REM and NREM and this classification was standardized by Allan Rechtschaffen and Anthony Kales in 1968 in R&K sleep scoring manual.
The different stages of sleep and its characters are assessed by polysomnography in a sleep specific laboratory. The sleep cycle is recorded by taking certain parameters like the electroencephalogram of the brain waves, electrooculography (EOG) of eye movements and the electromyography (EMG) of the activity of the skeletal muscles. In human beings each sleep cycle lasts for about 90-110 minutes at an average and each has a distinct physiological mechanism.
The American Academy of Sleep Medicine suggests that the NREM sleep consists of three stages which comprise the N1, N2, and the N3 stages. Little dreaming has been observed in this stage. N1 stage is also called as drowsy sleep and is accompanied by the alpha waves (8-13 Hz) to theta waves (4-7 Hz) of the brain. Jerks and hallucinations are often experienced by some people during this stage. N2 stage comprises sleep spindles (11-16 Hz) to K-complexes. The EMG depicts decreased muscular activity and consciousness and is 45%-55% of the total sleep in all individuals. N3 is characterized by presence of delta waves (0.5-2 Hz) and night mares, bed wetting, sleep talking and sleep walking have been found to be associated with this stage.The rapid eye movement (REM) comprises 20%-25% of the total sleep in all individuals. REM includes rapid movements of the eyes as well as rapid low voltage EEG.memorable dreaming has been observed in this stage. Muscular atonia has been observed in this stage and active physical movements are found in this stage.
Sleep is under the control of circadian clock and sleep-wake homeostasis. Circadian clock is the internal timekeeping, temperature fluctuating, enzyme controlling device that works with the neurotransmitter adenosine. High levels of adenosine are responsible for the sleepy behavior. In diurnal animals circadian clock is responsible for the secretion of a hormone termed as melatonin which causes sleepiness by decreasing the core temperature of the body. The timing of sleep is affected by chronotype which is the time interval between the structured and the restorative sleep. Awakening is controlled by circadian rhythm for example; a person who wakes up early in the morning will not be able to sleep more at the dawn. The circadian rhythm that controls the sleep duration is mediated by a gene DEC2. Mutation in this gene may lead to sleep of two hours less as compared to the normal individuals.
The optimal amount of sleep is affected by the concentration of the hormone melatonin and the core temperature of the body. Human sleep can vary with age. The deprivation of sleep is responsible for a number of symptoms that can badly affect human health. The important ones are irritability, in cognitive impairment, memory loss, hallucinations, impaired immune system, symptoms similar to ADHD. Others include risk of diabetes, increased heart rate, tremors and decreased temperature. Children need more hours of sleep in order to develop and function properly. The new born babies sleep for about 18 hours after which the sleep hours begin to decrease as age advances. New born babies spend 9 hours in REM sleep.
Sleep debt may be defined as the period of not getting enough rest and proper sleep which leads to mental, physical and emotional fatigue. The causes of irritability after sleep are yet not clear. The amount of sleep related behavior is under genetic control, mutation in BHLHB3 gene reduces the amount of sleep in normal persons from 8 hours to only 6 hours. A number of functions have been assigned for sleep for which a number of theories have been suggested. Wound healing has been observed to appear better during sleep. The immune system becomes stronger when proper sleep is taken. Sleep duration also affects somatic growth as the growth hormone secretion has been found to be increased during sleeplessness. Energy is conserved during sleep which is responsible for the better performance of the body. Studies have shown that the non-REM sleep is the anabolic state of body which is characterized by physiological processes of growth and rejuvenation of an organism’s nervous, muscular and skeletal systems. REM sleep is responsible for the development of brain.
Scientists have shown that sleep has a positive effect of memory processing also. Memory has been found to be affected by different stages of sleep such as the REM and slow wave sleep. Dreaming is an important part of sleep. Dreaming may be defined as the perception of sound and images during sleep. Dreaming is affected by pons and is observed during the REM phase of sleep. Many theories have been proposed for dreaming one such theory is the John Allan Hobson and Robert McCarley’s activation synthesis theory. This theory highlights that dreaming is associated with the random firing of the neurons in the cerebral cortex during the REM sleep. A number of substances are known to stimulate sleep like caffeine, cocaine and energy drinks.
It can be concluded that sleep is an important aspect of human life affecting the mental, physical and physiological aspects of human health.