Sleep is a state of physical and mental rest observed in all living beings. It is essential for survival and loss of sleep for a very long time may lead to physical and mental breakdown. Mind and body, which get tired after sustained activity, are rejuvenated after sleep. Therefore, in a normal living being, a few hours of sleep occurs naturally after vigorous activity, either physical or mental or both.
Physiology of Sleep
Though the body is at rest during sleep, there is considerable amount of eye movement occurring at this time. These are of two broad types and divide sleep into two stages:
- Rapid eye movement (REM) sleep and
- Non–Rapid eye movement (NREM) sleep.
REM Sleep is characterized by rapid movements of the eye and is the normal stage of sleep. It occupies 20-25% of total sleep lasting for about 90 – 120 minutes. Adults experience about 4 –5 periods of REM sleep during a normal night of sleep. However, it varies with age. At this stage the body is inactive, but the brain is active. Heart rate and breathing are irregular, that is, they are similar to that during waking hours. Dreams are found to occur at this stage. Certain neurons, known as REM sleep-on cells, have been found to be active during this sleep and these are mainly responsible for its occurrence. REM sleep is understood to serve certain functions for the organism.
- According to one theory, REM sleep helps in consolidation of procedural memory. In fact, a recent study showed improvement in the next day recall of memorized pairs of words through the artificial enhancement of REM sleep.
- According to a second theory, REM sleep is particularly important for the developing brain. This may be because it provides the neural stimulation that newborns need to form mature neural connections and for proper nervous system development. Studies on sleep deprivation have shown that deprivation of sleep in babies can result in behavioural problems, decreased brain mass and may result in permanent sleep disruption.
- A third theory argues that REM sleep is essential to shut down certain neurotransmitters in the brain, called the monoamines (norepinephrine, serotonin and histamine). This is called REM atonia, a state in which all motor neurons shut down and the body’s muscles don’t move. This helps the brain to regain full sensitivity. It fulfils important physiological needs vital for survival. Studies show that prolonged REM sleep deprivation has led to the death of experimental animals.
In NREM Sleep, the body is active, while the brain is inactive. It accounts for 75 - 80% of total sleep time in normal human adults. Dreams hardly occur at this stage. Unlike REM sleep, there are often limb movements and parasomnia sleep walking occurring in NREM sleep.
According to the American Academy of Sleep Medicine (AASM) standards, there are four stages in NREM sleep. These are differentiated only through EEG, unlike REM sleep, which is characterized by rapid eye movements:
- Stage N1 is sometimes referred to as somnolence or drowsy sleep, where the subject loses some conscious awareness of the external environment.
- During Stage N2 the conscious awareness of the external environment further disappears. It occupies 45 to 55% of total sleep.
- Stage N3 is considered part of the slow-wave sleep (SWS) and functions primarily as a transition into stage N4. This is the stage in which night terrors, bed wetting, sleep- walking etc. occur.
- Stage N4 is effectively a deeper version of N3, in which the deep-sleep characteristics are more pronounced. In fact, stages N3 and N4 are the deepest forms of sleep.
NREM sleep affects the body in several ways:
- Studies conducted on rats (Gumustekin et al.) have shown that wound healing is affected by sleep. The more an organism is deprived of sleep, the less healing of wounds was found to occur.
- The immune system and metabolism are also affected by NREM sleep deprivation (Zager, Andersen, Ruiz, Antunes and Tufik; Bonnet and Arand).
- Though it has yet to be clearly proven, some evidence are there to show that sleep, more specifically slow-wave sleep (SWS), has some effect on growth hormone levels (Van Cauter, Leproult and Plat).
- A strong correlation has been found between NREM sleep and higher-level cognitive functions such as decision-making, reasoning and episodic memory. Declarative memory and spatial memory are found to benefit more from early SWS-rich sleep (Born, Rasch and Gais).
Sleep and Memory
There is lot of evidence to show that sleep plays an important role in memory consolidation. Studies support a view of a “memory life-cycle”, which identifies three stages of memory processing:
- Stabilization – this may take as much as six hours. During this period, the memory appears particularly vulnerable to being ‘lost’.
- Consolidation – this occurs during sleep. At this stage reorganising of learnt material occurs.
- Re-consolidation – here memory is once again ready to be accessed and re-edited.
Studies on sleep deprivation and its effect on memory support the above theory. In a series of experiments, the brains of sleep-deprived and rested participants were scanned while performing complex cognitive tasks. Results showed that sleep-deprived participants performed worse than the rested participants. The MRI scan also confirmed less activity in the prefrontal cortex of the sleep-deprived participants. According to Dr Eve Van Cauter, professor of medicine at the University of Chicago, “We suspect that chronic sleep loss may not only hasten the onset but could also increase the severity of age related ailments such as diabetes, hypertension, obesity and memory loss.”
So, can one say that a period of sleep will help improve performance on memory tasks? Maybe ‘Yes’, and maybe ‘No’. While research supports the role of sleep in the consolidation of learned tasks in infants and children, there are so many other mechanisms, by which memory stores information, which need to be controlled before establishing a similar connection in adults.
Dreaming is the perception of sensory images during sleep and occurs during the REM phase of sleep. Here the dreamer perceives herself/himself more as a participant than an observer. A number of hypotheses have been proposed about the functions of dreaming:
- According to Sigmund Freud, dreams are the symbolic expression of frustrated desires that had been relegated to the subconscious. He used psychoanalysis to interpret dreams and uncover these desires.
- James Allan Hobson and Robert McCarley propose the activation synthesis theory of dream. According to this theory, there is random firing of neurons in the cerebral cortex during REM sleep. The fore brain then creates a story in an attempt to make sense of the nonsensical sensory information presented to it. This results in the many types of dreams.
- Recent theories of dreams place more emphasis on dreaming as a requirement for organisation and consolidation of recent memory and experience.
Amount of Sleep Required
What is the optimal amount of sleep required for a person?
The National Sleep Foundation (NSF) maintains that eight- nine hours of sleep is optimal for a normal human adult. A study conducted in the University of California, San Diego found that people who live the longest, sleep for six to seven hours each night. However, this is an empirically observed correlation, which is necessary but not sufficient condition for causality.
Studies conducted at the University of Warwick and University College London have found that lack of sleep can more than double the risk of death from cardiovascular disease, but then too much sleep can also double the risk of death. According to latest findings, a consistent sleep of about 7 hours per night is optimal for good health.
Children, however, need a greater amount of sleep than adults. For newborn babies, the average total hours of sleep required per day is 18 hours. This is mainly because the growth hormone secretions occur during sleep. As the child ages, a consistent decline in the hours of sleep is visible, with the 1 year child requiring 13-14 hours, 9 year child needing 9-10 hours and a 17 year adolescent needing 8-9.5 hours of sleep (according to the NSF) every night.
Somnipathy or sleep disorder is a medical disorder of the sleep patterns of a person or animal. The test commonly used for this purpose is the polysomnogram. As seen earlier, some of these disorders are serious enough to interfere with normal physical, mental and emotional functioning. Some common sleep disorders include:
- Bruxism: Here there is involuntary grinding or clenching of teeth while sleeping.
- Delayed sleep phase syndrome (DSPS): This is characterized by the inability to wake up and fall asleep at the desired times, but not the inability to stay asleep.
- Narcolepsy: This is the condition of falling asleep spontaneously and unwillingly at inappropriate times.
- Night Terror: Here the sufferer abruptly awakens from his sleep completely terrified.
- Periodic Limb Movement Disorder (PLMD): Sudden involuntary movement of arms and/or legs during sleep. This is also called nocturnal myoclonus.
- REM behaviour disorder: Here the person acts out violent or dramatic dreams while in REM sleep.
- Insomnia: This involves inability to fall asleep. At times when sleep occurs, there is the inability to stay asleep.
- Sleep apnea: Here there is obstruction of the airway passage during sleep, causing loud snoring and sudden awakenings when breathing stops.
- Sleepwalking or somnambulism: Here the person engages in activities that are normally associated with wakefulness, which may include walking, without the conscious knowledge of the person.
Common Causes of Sleep Disorders
The most common cause of a sleep disorder is some change in the life style, such as shift in work timings, shift in place of sleeping etc. Other problems that can create sleep disorder include:
- Poor environmental conditions: Excessive heat, cold, pollution noise bright light, loud noises can prevent sleep to a great extent.
- Stress: One of the main causes of stress is work. Extended working hours and the increased demands on worker productivity have resulted in less time for relaxation and socialisation. Other causes of stress are, conflicting choices in life, pressure of meeting a deadline, getting involved in too many things at a time, family problems, etc.
- Poor physical health: Chronic pain of back, neck or any other body-part has been found to prevent good sleep. Discomfort caused by any other physical problem, like a constricted air-passage may also result in disturbed sleep.
- Depression: This may result from a setback in life, it may occur because of loneliness, break in relationship or some long persistent physical or psychological problem.
- Drugs: Drugs like alcohol, coffee, tobacco, sleeping pills may affect the overall quality of sleep.
In order to prevent sleep disorders and enjoy a good night’s sleep, there are a number of things that one can do. Some of these include:
- Sticking to a regular sleep pattern
- Exercising regularly
- Using the bed only for sleeping and for no other purposes
- Avoiding alcohol, tobacco, caffeine and other recreational drugs
- Having a disturbance free environment while sleeping
- Listening to light music at bedtime
- Drinking warm tea made of herbs like, ginger, ginseng, chamomile before bed time
- Doing regular deep breathing exercises before sleeping, to relax the mind.
A variety of treatment options are available for patients diagnosed with a sleep disorder.
- Medicines, such as, Benzodiazepines, including diazepam, and lorazepam have been found to be effective in the treatment of PLMD and REM behaviour disorder.
- Sleepwalking and night terrors usually do not need to be treated in children. However, risk factors should be identified and minimized. In adults, drugs like Benzodiazepines may be required and can be lifesaving. It may also be used in the case of insomnia.
- For sleep apnea patients, relief comes with prescription of Continuous Positive Airway Pressure (CPAP). In this therapy, a steady stream of pressurized air is supplied through a tube connected to a plastic mask. This prevents the collapse of airway tissues during sleep.
- For other sleep disorders, like, DSPS, Narcolepsy, and those caused due to stress, light therapy treatment, as well as lifestyle changes including weight loss, quitting smoking or developing better sleeping habits may help in getting a good night’s sleep.
- HOW TO SLEEP WELL
- Memory Consolidation and REM Sleep
- Sleep & Stress
- The role of sleep in memory
- Experts challenge study linking sleep, life span
- Freud,S. The interpretation of dreams (1900). In The standard edition of the complete psychological works of Sigmund Freud.Vol. 3 London: Hogarth Press.
- Hobson,J.A., and McCarley, R. (1977). The brain as a dream state generator: An activation-synthesis hypothesis of the dream process. American Journal of Psychiatry, 134, 1335-1348.
- Gumustekin, K. et al. (2004). Effects of sleep deprivation, nicotine, and selenium on wound healing in rats [Electronic version]. Neuroscience, 114, 1433-1442.
- Jenni, O. G., Molinari, L., Caflisch, J. A., and Largo, R. H. (2007). Sleep duration from ages 1 to 10 years: Variability and stability in comparison with growth [Electronic version]. Pediatrics, 120, e769-e776.
- Born, J., Rasch, J., and Gais, S. (2006). Sleep to remember [Electronic version]. Neuroscientist, 12, 410.
- Van Cauter, E., Leproult, R., and Plat, L. (2000). Age-related changes in slow wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men [Electronic version]. Journal of the American Medical Association, 284, 861-868.
- Zager, A., Andersen, M. L., Ruiz, F. S., Antunes, I. B., and Tufik, S.(2007). Effects of acute and chronic sleep loss on immune modulation of rats [Electronic version]. Regulatory, Integrative and Comparative Physiology, 293, R504-R509.
- Bonnet,M. H. and Arand, D. L. (2003). Insomnia, metabolic rate and sleep restoration [Electronic version]. Journal of Internal Medicine, 254, 23-31.