INTRODUCTION:

Human performances are interdependent on sleep-wake rhythm and are naturally functional in adaptation to day-night differences in the environment¹. Sleep is a naturally recurring circadian cycle, believed to be a restorative process essential for body restitution, facilitation of motor function, and consolidation of learning and memory^2,3. Empirical evidencesshow that sleep has profound impacts on mood, memory, cognitive ability, working performance, psychomotor skills and overall human behaviour^4,5.

There is a universal acknowledgement thathealthy adults require approximately eight hours of sleep each night for full cognitive functioning. An individual’s quality of life can be disrupted for sleep deprivation, sleep-loss and poor sleep hygiene. Need for sleep varies noticeablyamong individuals ranging from 7 to 8.5 hours per day⁶. Two processes, namely homeostatic process ‘S’ and circadian process ‘C’ are regulated by sleep⁷. The homeostatic process (S) is subject to sleep and wakefulness whereas circadian process (C) is subject to endogenous circadian pacemaker affecting the onset and offset of human sleep episode. The interaction of these two processes determines the sleep-wake cycle⁷.

Studies on sleep deprivationare designed into two formats to measure the effects of sleep loss. In total sleep deprivation (TSD)format, often called acute sleep deprivation (ASD), subjects are kept awakened constantly for 24–72 hoursand are deprived of all stages of sleep. In partial sleep deprivation (PSD), subjects are restricted to sleep time for 2-6 hours after being deprived of 1-2 stages of sleep including Rapid Eye Movement (REM) sleep during several consecutive nights. There is an extensive literature documenting the negative cognitive consequences of PSD and TSD^8-21.

studies on SD and its mechanisms:

There are two general approaches taken by theories of sleep, firstly, restoration theories, and secondly the evolutionary or ecological theories.Restoration theories suggest that sleep exists for repair and restore the body^22,23. This theory is further supported by McGinty,andSzymusiaka (1990)²⁴, Cirelli, Gutierrez, and Tononi (2004)²⁵. The theory claims that Non-REM (NREM) sleep restores biological processes that deteriorate during the day whereas REM sleep replenishes and renews brain processes through the process of protein synthesis. However, contradiction was found by Horne (1988)²⁶, reported that amino acids, build in to proteins, are only available for 5 hours after eating, therefore for most people they wouldnot be available during sleep, suggesting that protein synthesis cannot occur during sleep, and thus casting doubt on Oswald’s theory. Support for Horne comes by the study of Stern and Morgane (1974)²⁷who found that neurotransmitter levels’ are replenished during REM sleep, backed up by the fact that antidepressants increase neurotransmitter levels’, reducing REM activity. Further support comes by Hartmann (1973)²⁸ whoreported that REM sleep is a time for synthesizing noradrenaline and dopamine to compensate for the amount used during the day. It is also evident that the patients with brain trauma spend an increased amount of time in REM sleep, suggesting that it is the increased blood flood that occurs during REM sleep that actually aids brain repair, and restoration. Quadriplegics still spend similar proportions of sleep in NREM (including stages 3 and 4) as able bodied people despite the obvious lack of the need for physiological restoration²⁸.

Contemporary understanding of the physiological patterns of sleep and wakefulness has shown that different brain-processing networks and neurochemical systems are involved in both states²⁹. REM sleep is generated by complex neuronal interactions within the pontine reticular formation, whereas NREM sleep arises from activities in the thalamocortical network, which are specifically implicated in different aspects of long-term memory systems. Significance of sleep spindles and K-complex are of great importance in sleep. Both they have role in the onset of stage-2 sleep (10-12 Hz). Sleep spindles often called sigma waves generated in the reticular nucleus of thalamus have been shown in aiding sleep in the occurrence of disruptive external stimulus like sound and touch in order to maintain tranquility³⁰. Sleep spindles are further associated with the integration of new information into existing knowledge³¹. Two kinds of spindles like fast and slow spindles are associated with succeeding steps in sleep-dependent memory processing. The fast spindles coincides with hippocampal sharp wave facilitates the transfer of memory-related information from the hippocampus to the neocortex, andslow spindles are thought to be important in formation of neocortical long-term memory representation³². K-complexes have been suggested to protect sleep in addition to engage in information processing, an essential part of the synchronization of NREM sleep. They are also responsive to internal and external stimuli in a reactive manner³³. Another suggested function of K-complexes is aiding the activation homeostasis of synapses and memory consolidation³⁴. The restorative function of sleep may be a consequence of the enhanced removal of potentially neurotoxic waste products that accumulate in the awakened central nervous system. In recent study,it was found that, the brain utilizes sleep to flush out waste toxins. This waste removal system, called glymphatic system is letting fluid flow rapidly through the brain, is one of the major reasons why we sleep³⁵.

Evolutionary theories often called as ecological theoriesurge on the relation of sleep with the ability to survive³⁶. According to this approach, the function of sleep serves as an adaptive purpose and sleeping patterns have evolved naturally in order to promote survival. Webb’s ‘hibernation theory’, suggested that the purpose of sleep is for energy conservation. Small animals have a high metabolic rate and hence use more energy, so they need more sleep³⁷. For these animals sleep is a period of enforced inactivity, allowing them to conserve their energy to fit with their ecological niche. This theory helps to explain the correlation between body mass and sleep length in mammals as the little brown bat sleeps for 19 hours whereas the elephant sleeps for only 3 hours. However, exceptions are found by Allison, and Cicchetti (1976)³⁸ that large creatures as sloths sleep for 20 hours a day, thus undermines the evolutionary theory as an explanation of sleep. They also discovered that larger animals had less NREM sleep but not less REM sleep, which relates to the fact that during NREM sleep the energy consumption of the brain drops, whereas in REM sleep the brain remains relatively active. This shows that NREM sleep is more important for energy preservation, thus supports evolutionary explanations. Furthermore, it has been suggested that REM sleep may have evolved later to maintain brain activity, thus supporting the greater need for REM sleep in infants whose brains are developing. Webb’s ‘predation theory’ explained thatsleep constrains the risk of predation, as prey species like gazelle must remain awake and vigilant to avoid predators, and for that, they sleep for reduced amount of time. Predator like tiger can sleep for longer time as they are not at threat to be preyed and does not need to adapt to the environment in terms of altering sleeping patterns to aid survival³⁹ coined the ‘waste of time’ hypothesis, and claimed that sleep only helps animals to stay out of the way during times when they are most vulnerable, whereby they sleep in darkness and hidden places to avoid get preyed. Allison, and Cicchetti³⁸ claimed that the species who has a higher risk of predation do actually sleep less, again there are exceptions that rabbits who are at great risk to get preyed sleep as much as moles who are at a much lower risk. Yet, being awake is riskierand sleeping keeps them out of danger^40,41. The evolutionary approach fails to explain why we have a strong drive for sleep when being sleep deprived, and why there are different stages and cycles of sleep. Although both theories make insightful arguments in understanding the function of sleep,but because of the weaknesses, Horne (1988)⁴² proposed a theory ‘combined approach’ that combined both the restorative and evolutionary theories to explain the functions of sleep. He stressed on two types of sleep, core sleep and optional sleep. Horne proposed that the restorative approach can explain why we need core sleep (stage 4), whilst the optional sleep (stage 1-3) can explain why we might sleep to waste time and conserve energy. However, it could be concluded that sleep has more of an influence on the brain. Both theories have had practical applications. Lastly, there are issues of extrapolation and limitations, that not all of the findings can be applied to humans due to variations in sleeping patternsalong with cultural and environmental influences⁴².

Sleep loss or prolonged wakefulness pose adverse effect on cognitive performances.The theorydeals with two main approaches (1) SD has general effects on alertness and attention, and (2) SD has selective effects on certain brain structures and functions. Individual differences in the effects are also in consideration. Cognitive impairments are ensued through decreased alertness and attention, lapses, slowed responses, and wake-state instability. The main reasons for the decrease in cognitive performances resulted from SD are attentional lapses and brief moments of inattentiveness¹¹. The lapses occur asmicrosleepsare characterized by very short EEG wave like short periods of sleep-like activity⁴³. Itwas assumed that cognitive performances remain intactin-between the lapses, but the slowing of cognitive processing has also been observed independent of lapsing¹¹. The hypotheses addresses that performances after SD most likely deteriorate in long, simple, and repetitive tasks required reaction speed or vigilance. In addition, fluctuations in alertness and effort are also observed. These fluctuations lead to variation in performance as the wake-state instability hypothesized¹⁹. According to selective impact theory, SD interferes with the functioning of certain brain areas which ultimately impairs cognitive performance,often referred to ‘sleep-based neuropsychological perspective’¹³. Horne’s (1993)⁴⁴ prefrontal vulnerability hypothesis is the most conceivable theory in this category, theorizes that SD especially impairs cognitive performances depended on prefrontal cortex isresponsible for higher functions like language, executive functions, divergent thinking, and creativity. The theory opines that tests should be complex, new, and interesting to see the SD effect since cognitive flexibility and spontaneity are required for a good performance. The theory further assumes the deterioration of subjects’ performances in simple and longtasks is simply due to monotony^21,^45-46. These studies focused in measuring working memory or other attentional functions with the type of tasks that are not traditionally emphasized in prefrontal vulnerability hypothesis⁴⁷. Inter-individual differences are also important in reaction to SD since there are individual differences in length, timing and structure of sleep. It is also evident some people are more vulnerable to sleep loss than others⁴⁸.

Patrick and Gilbert (1896)⁴⁹were the pioneers in reporting the performance impairing effects of total sleep deprivation (TSD).They reported impairments in reaction time, discrimination time, visual acuteness, sound discrimination, letter naming, mathematical addition, and memory during TSD. Since then, many researchers have replicated and extended these findings which support that TSD impairs many different brain functions including vigilance, working memory, short term memory, executive function, math processing, cognitive speed, spatial orientation, and motor control ^50-56.

Attention and working memory are interrelated. Attention and working memory both are linked to the functioning of frontal lobes⁵⁷. The ability to selectively process information is called attention, and to retain information in an accessible state is called working memory are critical aspects of human cognitive capacities. Although the nature of the relationship between these two constructs is not well understood, but attention and working memory were found to interact closely during encoding and manipulation⁵⁸. In addition, only central attention was found to be necessary for manipulating information in working memory⁵⁸. In measuring divided attention, performance was impaired after 24 hours of SD⁵¹. Detrimental effects on both speed and accuracy in measuring attention and working memory during SDwere observedpreviously and^14-18 speed-accuracy trade-off phenomenon in attentional task where compromising speed in concentrating to accuracy and vice-versa was also observed⁵⁹. Working memory is a vital mental ability that is compromised following SD.Working memory has four subsystems: phonological loop, visuospatial sketchpad, episodic buffer and central executive^60,61. Certain attentional functions like sustained attentionor vigilance is the executive processes of working memory⁶². Psychomotor skills are sensitive to the effects of sleep loss concerning both reaction speed and lapses⁶³. Contrarily some studies found no difference after one night of SD between deprived and non-deprived subjects in simple reaction time, vigilance or selective attention tasks⁶⁴. Performance on the Wisconsin Card Sorting Test, a measure of frontal lobe function, also remained even^64,65.

Studies measured long-term memory with a variety of tasks were found somewhat inconsistent in their results. In verbal episodic memory, SD of 35 hours impaired free recall but not recognition²⁰, whereas the results were opposite with one night of SD ⁶⁴. Both studies were conducted with small sample (n=9-13) size, offers a possible explanation for the variation in results. Moreover, Drummond and colleagues adopted a within-subject design, whereas Forest and Godbout ⁶⁴adopted between-subject design²⁰.

In visual memory, recognition was similar in the experimental and control groups^66,21. Studies where subjects faced only one night SD under controlled conditions have failed to find any effect^67,68. Impaired performance was observed in probed forced memory recall⁵¹, and memory search⁶⁹. No effect was found in episodic memory⁶⁷, and prose recall, implicit memory, crystallized semantic memory, procedural memory or face memory⁶⁸.

Sleep deprivation impairs visuomotor performance because iconic memory has short duration and limited capacity¹². Sleep deprivation impedes engagement of spatial attention⁸. Decreased oculomotor functioning is also associated with impaired visual performance⁷⁰, and sleepiness^70,71. However, inconsistencies in results are also found in studies onoculomotor impairment with cognitive performance decrements⁶⁸.

Inconsistent results were observed in reasoning ability.Deteriorated performance was reported^46,72. No effects were reportedin other studies^68,73. Deterioration in decision-making also appears as more variable performance in applied strategies⁷⁴, as well as in more risky behaviour⁷⁵.

Motor function, receptive and expressive speech, and memory deteriorated after one night of SD, whereas tactile function, reading, writing, arithmetic and intellectual processes remain intact⁷⁶. The adverse effects of TSD in real-life settings, like health care workers, military personnel and professional drivers are also well understood^4,77,78. Sleep deprivation impairs self-evaluation in cognitive performance. During 36 hours SD, the subjects were found confident of the correctness of their answers²¹,thoughtheanswers were wrong.

Very few studies have attempted in comparing the effects of TSD (≥24 hours) and chronic PSD⁷⁹. They found both speed and accuracy in the psycho-visual test (PVT) deteriorated almost linearly as the sleep restriction continued.Based on existing literature, it is difficult to compare the effects of TSD and PSD for the large variation in methodologies including the length of SD and the type of cognitive measures.In well-controlled studies, it has been observed that the less sleep obtained due to sleep restriction, the more cognitive performance is impaired^79-81. Table 1 summarized the cognition in which deterioration of performances have been reported during total sleep deprivation (TSD).

Table-Cognitive tests in which deterioration of performances have been reported during total sleep deprivation (TSD).

Type of Measures	Cognitive Tests	Result	Authors
Visuomotor performance	Critical tracking	Performance declined	Van Dongen et al., 2004
	Letter cancellation task (visual search)	Performance declined	Casagrande et al., 1997; De Gennaro et al., 2001
	Trail-making task	Performance declined	Wimmer et al., 1992
	Maze tracing task	Performance declined	Blatter et al., 2005
	Digit symbol	Performance declined	Van Dongen et al., 2003, 2004
	Digit symbol, Bourdon-Wiersma, other psychomotor tests	No significant difference was found	Quigley et al., 2000; Alhola et al., 2005
	Procedural motor task	Performance declined	Forest & Godbout 2000
Attention	Simple reaction time, Reaction speed and vigilance	Performance declined	Choo et al., 2005; Karakorpi et al., 2006
	Choice reaction time tasks	Performance declined	Wilikinson et al., 1990; Smulders et al., 1997; Wright &Badia, 1999; Frey et al., 2004; karakorpi et al., 2006; Kendall et al., 2006
	Serial reaction time test	No significant difference was found	Nilsson et al., 2005
	Vienna Test System (computerized): Vigilance, simple reaction time)	No significant difference was found	Lee et al., 2003
	Cognitrone (visual analytical ability, attention and working memory vigilance	Performance declined	Wu et al., 1991, Corsi-Cabrera et al., 2003, Karakorpi et al., 2006, Sagaspe et al., 2006, Taillard et al., 2005
	Flanker task (computerized: attention, vigilance?)	Performance declined	Tsai et al., 2005
	Dichotic listening (vigilance)	Performance declined	Johnsen et al., 2002
	Psychomotor vigilance task (PVT)	Performance declined	Dinges et al., 1994; Wright &Badia, 1999; Doran et al., 2001, Van Dongen et al., 2003; Frey et al., 2004; Graw et al., 2004; Van Dongen et al., 2004; Adam et al., 2006; Blatter et al., 2006
	Serial addition and/or subtraction task	Performance declined	Drummond et al., 1999; Thomas et al., 2000; Van Dongen et al., 2003 & 2004; Kendall et al, 2006
	Two column addition	Performance declined	Wright &Badia, 1999; Frey et al., 2004
	Visuo-spatial attention (saccadic eye movements)	Performance declined	Bocca& Denise, 2006
	Finding Embedded Figures Test	Performance declined	Blagrove et al., 1995
	Auditory attention task	Performance declined	Blagrove et al., 1995; Linde et al., 1999
	Dual task	Performance declined	Wright &Badia 1999; Frey et al., 2004
	Dual task	No significant difference was found	Drummond et al., 2001; Alhola et al., 2005
	Paced Auditory Serial Addition Test (PASAT)	No significant difference was found	Binks et al., 1999
Decision making	Critical reasoning, masterplanner	Performance declined	Harrison & Horne, 1999
Decision making	Decision-making task	Performance declined	Linde et al., 1999; Kilgore et al., 2006
Working memory	N-back	Performance declined	Smith et al., 2002; Choo et al., 2005
	LTR, PLUS	Performance declined	Chee&Choo, 2004, Chee et al., 2006
	PLUS-L (verbal working memory)	Performance declined	Chee at al., 2006
	Delayed-match-to-sample task	Performance declined	Habeck et al., 2004
	Choice-reaction time task (with working memory component)	Performance declined	Jennings et al., 2003
	Brown-Peterson	Performance declined	Forest & Godbout, 2000
	Stemberg verbal working memory task	Performance declined	Mu et al., 2005
	Working memory task	Performance declined	Wimmer et al., 1992
	Digit recall	Performance declined	Frey et al., 2004
	Digit span	No significant difference was found	Linde& Bergstrom, 1992 (2 studies); Quigley et al., 2000
	Word recall (working memory)	No significant difference was found	Quigley et al., 2000
	Verbal working memory, visuo-spatial working memory test	No significant difference was found	Nilsson et al., 2005
	Spatial working memory task	No significant difference was found	Heuer et al., 2005
	Attentional power (effortful information processing)	No significant difference was found	Linde& Bergstrom, 1992 (2 studies)
Long-term memory	Word memory test	Performance declined	Drummond et al., 2000
	Temporal memory for faces (recency)	Performance declined	Harrison & Horne, 2000
	Probed forced memory recall and digit recall	Performance declined	Wright &Badia, 1999
	Memory search	Performance declined	McCarthy & Waters, 1997
	Paired word learning (implicit memory)	Performance declined	Forest & Godbout, 2000
	Episodic memory (Claeson-Dahl test)	No significant difference was found	Nilsson et al., 2005
	Implicit memory test, prose recall, Mill Hill vocabulary test (crystallized semantic memory), procedural memory, face memory)	No significant difference was found	Quigley et al., 2000
	Benton visual retention test	No significant difference was found	Alhola et al., 2005
Verbal functions	Logical reasoning	Performance declined	McCarthy & Waters, 1997
	Logical reasoning test (Bladdeley)	Performance declined	Blagrove et al., 1995; Monk & Carrier, 1997
	Logical reasoning test (Bladdeley)	No significant difference was found	Linde& Bergstrom, 1992 (2 studies); Quigley et al., 2000, Drummond et al., 2004
	Word detection task, repeated acquisition of response sequence task	Performance declined	Van Dongen et al., 2004
	Vowel/consonant discrimination task, letter recognition task	Performance declined	Wimmer et al., 1992
	Sentence processing, categories test, spot the word, word recognition	No significant difference was found	Quigley et al., 2000
	Word fluency, Booklet form of the category test	No significant difference was found	Binks et al., 1999
Response inhibition	Response inhibition (the Haylings sentence completion task), verb generation to nouns	Performance declined	Harrison & Horne, 1998
	Go-NoGo (response inhibition)	Performance declined	Drummond et al., 2006
	Stroop (color-word, emotional, specific)	Performance declined	Sagaspe et al., 2006
	Spatial Stroop (suppression of prepotent responses)	Performance declined	Heuer et al., 2005
	Stroop	No significant difference was found	Binks et al., 1999
Other measures	Dichotic temporal order judgment	Performance declined	Babkoff et al., 2005
	Negative priming (effect vanished during SD)	Performance declined	Harrison &Espelid, 2004
	Task-shifting	Performance declined	Heuer et al., 2004 (2 studies)
	Simon task	Performance declined	Heuer et al., 2005
	Raven’s progressive matrices	Performance declined	Linde& Bergstrom, 1992
	Figural form fo the Torrance Tests of Creative Thinking	Performance declined	Wimmer et al., 1992
	Modified Six Elements test (story-telling, simple arithmetic calculations and object naming)	Performance declined	Nilsson et al., 2005
	Switching task	Performance declined	Frey et al., 2004
	Implicit sequence learning in the serial reaction task	Performance declined	Heuer et al., 1998; Heuer& Klein 2003
	Explicit sequence learning task (serial reaction tasks)	No significant difference was found	Heuer et al., 1998
	Luria-Nebraska Neuropsychological Battery, Calculation and digit span from WAIS	Motor, rhythm, receptive & expressive speech, memory and complex verbal arithmetic function were decreased, but no differences in distractibility, tactile function, visual function, reading, writing, arithmetic and intellectual process function were observed	Kim et al., 2001
	Number-series inductions	Performance declined	Linde& Bergstrom, 1992
	Novel oddball task (auditory)	TSD may compromise cognitive functioning in different regions of the brain	Gosselin et al., 2005
	Random generation tasks	TSD does not impair all sorts of executive functions, but only some.	Heuer et al., 2005 (3 studies)
	Complex navigation task	No significant difference was found	Strongman et al., 2005
	Wisconsin Card Sorting Test (computerized), WAIS-R short form	No significant difference was found	Binks et al., 1999

Abbreviations: SD: Sleep deprivation; WAIS: Wechsler Adult Intelligence Scale; WAIS-R: Wechsler Adult Intelligence Scale-Revised; TSD: total sleep deprivation.

METHODOLOGICAL CONCERNS:

Sleep deprivation studies have included several domains in measuring cognitive performances. The most exhaustively evaluated performances of SD researches include different verbal functions, decision-making, attentional functions, working memory, long-term memory, visuomotor and psychomotor skills^{4,8,11,13-18,20,43,57,60,64,70,71,75,77,78,82-84}. These results, however, may be partially biased due to small sample (9-29) sizes^64,65, insufficient control of the subjects’ sleep history or the use of stimulants before the study^{14,16,66,80,85}. In some studies,performance was impaired after 24 hours of SD ⁵¹, whereas performance was maintained after 25-35 hours of SD^66,86. The uneven loads between different subtests and uncontrolled practice effect might cause the divergent findings in these studies⁸⁷. In addition, differences in essential study elements, for instance the age and gender of subjects, and the duration of SD add complications in comparison of the results. In measuring attention or working memory, speed and accuracy are the two important aspects of performance. Due to speed-accuracy trade-off phenomenon,individuals may tend to switch their emphasis between the two with attentional focusing ⁵⁹. In some studies speed was found impaired while accuracy remained intact^15,70. Opposite results are also found in other studies^76,88. Yet again, many studies depicts detrimental effect on both speed and accuracy^14-18. The speed-accuracy trade-off phenomenon is reasonably affected by gender, age, and individual differences in response style^82,83, a potential reason for inconsistencies in the SD results.

The adverse effects of sleep deprivation on cognitive performances are demonstrated, but there are studies that failed to detect any deterioration^{14-18,20,21,64-68,70,73,76,86,88}.Study on SD often may lead to researcher to compromise in the study design since it is quite laborious and expensive⁸⁷. A small sample size can reduce the statistical power. On the other hand, a larger sample may come at the expense of different methodological, practical and feasibility disputes. The common drawback in SD studies is recruitment of samples⁸⁷. Usually, subjects are favoured to be drawn via advertisement or from clinics.Strict exclusion criteria regarding subject’s physical and mental conditions, sleeping habits, sleeping and cognitive problems are essential⁸⁷. Additionally, use of medication and sympathetic drug or stimulants like caffeine, alcohol, tobacco should be controlled before the experiment^{14,16,66,80,85}. Subjects must not beany substance user because taking attempt to prohibit the use of stimulants may increase their withdrawal symptoms from study.Sometimes it is difficult to specify exactly which cognitive function is used in certain performance because of cognitive processes having intertwining characteristics⁸⁷.

Lack of fundamental research with larger sample size^19,20,64as well as unavailability of empirical data on sample of other parts of the world has made the results ofsleep-cognition study a challenged one. In addition,methodological shortcomings such as cognitive measures of different sensitivity, insensitivity of some measures, failure in controlling practice effect, individual subject characteristics, inadequate description of study protocols and other confounding variables like individual sleep case-history, napping or snoozing during study, sleeping in unfamiliar or differentsettings, adaption night in the lab, and leaving the subjects in their normal choice before the study commences have made it difficult to interpret the neutral results⁸⁷.

CONCLUSION:

The review of sleep deprivation and cognitive performances revealed that there is considerable relationship between sleep and cognitive performances. Attention, working memory, long-term memory, visuomotor performances, decision making etc. have found to be deteriorated in sleep deprivation ^14,18,20,89. Yet, there remain controversies due to methodological concerns⁸⁷. One of the important issues that we have noticed is that most of the studies on sleep and cognitive performances have been conducted in western culture ^90-92. Therefore, we assume that these studies are questioned to cultural and climate biasness. We could not find any single research conducted in Eastern and Asian context regarding sleep deprivation’s effect on cognitive performances. However, we strongly believe that systematic studies should be conducted in eastern and Asian context considering the cultural and climatic variations. Although sleep studies have achieved some remarkable developments due to incorporation of wide variety of new and sophisticated technology, it is not only the technology; society itself is becoming increasingly globalized that leads psychologists to understand the cognition and behaviour in different cultural contexts. Many early scientists and philosophers, for example, Wilhelm Wundt (1916) ⁹³recognized the importance of studying both cognitive processes and cultural diversities in order to understand human behaviour and cognition.

Humans, as a species, are capable of surviving in extremely diverse environments because their behaviours and mental abilities are flexible largely. Instead of having most behaviour hardwired into the organism, human brains are much flexible and capable in coping with the changing environment^94,95,96. This flexibility of cognition and behaviour allows human beings to benefit from the collectively shared experience and knowledge accumulated by preceding generations. Through the essentially automatized absorption of surrounding customs and behaviours humans are able to acquire one or more of an enormous range of cultural tool kits, and can unthinkingly adapt to any practical situation.

Evidence is mounting that executive parts of the brain play a large role in the “top down” parsing and interpretation of sensory information⁹⁷. Expectations and goals, which are shaped by culture, play a role in guiding the neural processing of sensory information in sorting through and deciding at a neural level which information is attended to and which is disregarded⁹⁸. For example, the ventral visual processing areas commonly associated with the interpretation or meaning of visual input⁹⁹, as well as some related to attentional control¹⁰⁰ show measurable differences in how stimuli are processed between East Asian and Western populations. Thus, over time, a person’s behaviour and cognition becomes intrinsically biased towards the patterns, priorities, and connections that are frequently repeated within their social circles¹⁰¹. We, therefore, suggest systematic studies on sleep and cognitive performances in eastern, Asian, and African cultures considering the above-mentioned issues.

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Received on 24.11.2017 Modified on 18.01.2018

Research J. Pharm. and Tech 2018; 11(8): 3658-3666.

DOI: 10.5958/0974-360X.2018.00673.X