The Physiological Role of Sleep
Sleep is an active state of unconsciousness that is regulated in a complex manner and is vital for our health and wellbeing.1 During sleep, the chemical reactions (metabolism) of the body slow down, and this slowing down helps to conserve energy. Processes that are important for the recovery of the body — such as the synthesis of proteins, and tissue growth and recovery — are also prominent during sleep. Moreover, sleep is involved in the modulation of the immune response and regulation of body temperature, cognition, and mood.2,3
Sleep and Cognition
Cognition represents the mental processes that are implicated in the acquiring and processing of knowledge and comprehension. Core cognition processes include attention, memory, executive functioning, language, and visual-spatial perception.4 It is well established that sleep plays an important role in several different aspects of cognition. Sleep facilitates memory consolidation, which is the transformation and stabilization of short-term memories into long-term memories.5 However, sleep also positively affects the learning and encoding of memories after awakening.6 Moreover, sleep facilitates gaining insight, integrating information, and building connections among learned information.7 It also modulates and optimizes working memory and decision-making.8,9
Sleep Deprivation and Its Negative Health Consequences
Adults need at least 7 hours of sleep per 24 hours to maintain good health and alertness. Beside the quantity, a good quality of sleep is also important for its full health benefits. When people cannot get the necessary amount of sleep, they become sleep-deprived. A variety of reasons may lead to sleep deprivation, such as lifestyle choices, shift work, and medical conditions. Sleep deprivation may be either total sleep deprivation (when a person has not slept at all for a certain period of time) or chronic sleep restriction (when the sleep duration has been shorter than the recommended duration each night).10
Sleep deprivation is a wide-spread phenomenon in modern society. It may be partially driven by the high demands on our time, which may lead us to try and fulfill all our obligations by cutting down our sleeping time. However, sleep deprivation may have serious health consequences and has even been associated with increased all-cause mortality.11 Sleep deprivation also leads to an increased risk of obesity, diabetes mellitus, high blood pressure (also known as hypertension), heart disease, and stroke. Moreover, sleep deprivation negatively affects cognition and mood.
Sleep deprivation has been associated with increased daytime sleepiness and decreased alertness, which increase the risk of accidents and errors.12,13 This is especially problematic if sleep-deprived individuals have to drive, operate machinery, or make important decisions. (One way to temporarily deal with this problem is with hydrogen tablets.14)
Sleep Deprivation and Its Negative Effects on Cognition
The first study, which reported negative effects of total sleep deprivation on attention and memory in humans, was published at the end of the 19th century.15 These findings have been confirmed by numerous studies and extended to different paradigms of sleep deprivation. Thus, the negative effects of sleep deprivation on human cognition are very well established.3
More up-to-date studies have focused on the cognitive effects of total sleep deprivation than on the effects of chronic sleep restriction, likely because total sleep deprivation is easier to model under experimental conditions. Overall, both the severity of sleep deprivation and the number of days over which it extends influence its effect on cognition. In one study, cumulative additional wakefulness of over 15.84 hours was associated with errors in alertness under conditions of both total acute sleep deprivation and chronic sleep restriction.16 In the same study, the authors found that sleep-deprived individuals may underestimate the negative cognitive consequences of sleep deprivation.16
These results underscore how important it is to raise the awareness of the negative effects of sleep deprivation on cognition. However, in another study the speed of sleep deprivation affected the development of cognitive deficits, and “rapid” sleep loss was associated with more pronounced cognitive defects than “slow” sleep loss.17 The authors interpreted this effect as likely being caused by a compensatory mechanism that is activated under conditions of chronic sleep restriction.
The Effects of Sleep Deprivation on Specific Aspects of Cognition
Sleep deprivation affects different aspects of cognition, including attention, working memory, short-term memory, learning, and decision-making.
The Effects of Sleep Deprivation on Attention
The aspect of cognition that is apparently most profoundly affected by sleep deprivation is attention.18 The negative effect of sleep deprivation on attention has been confirmed using different testing paradigms for both total sleep deprivation and chronic sleep restriction.10,19 One of the aspects of attention that sleep deprivation influences negatively is vigilance (the state of being alertly watchful). For example, chronic sleep restriction worsened the performance on a psychomotor vigilance task, which measures how fast a person responds to a visual stimulus.19
Sleep Deprivation and Working Memory
Working memory represents the processes that control, maintain, and regulate information relevant to the performance of different tasks. Sleep deprivation negatively affects working memory,20 and imaging studies have shown changes in the activity of certain brain areas that are associated with these effects.21
Sleep Deprivation and Learning
Sleep deprivation has been correlated with poor learning. As sleep deprivation is a widespread phenomenon among students of all education levels, ensuring sufficient quantity and quality of sleep may help students boost their academic performance.22
Sleep Deprivation and Decision-making
Sleep deprivation negatively affects certain types of decision-making, such as decisions involving unexpected changes and uncertainty. This is at least partially due to the fact that sleep-deprived individuals react to feedback in a blunted manner, which impedes their adaptation to a changing environment. This may lead to very serious mistakes in a real-world crisis situation that requires a prompt response.23
Biological Mechanisms That Underlie the Negative Effects of Sleep Deprivation on Cognition
Sleep deprivation affects cognition by both generally influencing attention and alertness and specifically modulating certain brain regions.10 The negative effect of sleep deprivation on attention and alertness is mediated through lapses. Lapses are brief periods of inattentiveness that are caused by microsleeps. In turn, microsleeps are short periods with brain wave activity that resembles a sleep state.24
The effects of sleep deprivation on specific brain areas have been investigated with magnetic resonance imaging (MRI) and positron emission tomography (PET).21 A PET study found that total sleep deprivation decreases the brain glucose metabolism (which is an indicator of brain function) by 6%–8%. This effect was especially pronounced in the frontal and prefrontal cortex and thalamus, which are brain areas important for higher-order complex cognitive functions and for the regulation of attention.25 However, MRI studies found that functional brain changes after total sleep deprivation may depend on the performed task. These findings suggest the possible involvement of compensatory mechanisms, such as the recruitment of collateral brain areas.
Sleep deprivation also leads to molecular changes in the brain cells, which underlie the associated changes in cognition. Thus, sleep deprivation decreases long-term potentiation, which is a process of strengthening the connections between neuronal cells by repeated activation. Long-term potentiation is very important for learning and memory. The effect of sleep deprivation on long-term potentiation is thought to be mediated by changes in molecules within the neuronal cells, on their surface, or released by neuronal and other brain cells. These molecules include receptors for neurotransmitters (chemical messengers between neurons), molecules that modulate the growth of neuronal cells, and intracellular signaling molecules.26
The identification of the molecular mechanisms that underlie the effects of sleep deprivation on cognition has been impeded by the wide range of experimental paradigms of total sleep deprivation and chronic sleep restriction and their variable duration. However, despite these challenges, the understanding of the neurobiological mechanisms underlying the negative cognitive effects of sleep deprivation has advanced greatly.
Recovery from Negative Cognitive Effects Caused by Sleep Deprivation
Recovery sleep can reverse the negative cognitive effects of sleep deprivation. Nevertheless, the underlying mechanisms are still being investigated, and the exact duration of necessary recovery sleep to restore cognitive functions after sleep deprivation has not been definitively determined. A recent study found that one night of total sleep deprivation significantly impaired the connectivity between the hippocampus and multiple brain areas and memory performance in a specific task. Two nights of recovery sleep restored the hippocampal connectivity but could not completely restore the memory performance.27 Interestingly, some studies have shown that a longer period of recovery sleep may be necessary to reverse the cognitive effects of chronic sleep restriction than those of total sleep deprivation.10 Furthermore, interindividual differences may also play a role in the speed of cognitive recovery after sleep deprivation.
Healthy Sleep Habits and Reversing the Negative Effects of Sleep Deprivation on Cognition
The negative effects of sleep deprivation on cognition may lead to serious errors with long-term consequences, for example when driving, operating machinery, or making important decisions. Therefore, raising the awareness of the possible consequences of sleep deprivation and education on healthy sleep habits should be priorities.
Recovery sleep would help sleep-deprived individuals reverse the negative cognitive effects of sleep deprivation toward baseline. Then, depending on the reasons for sleep deprivation, a long-term strategy should be developed to ensure sufficient quantity and quality of sleep. There are numerous publicly available sources of information on healthy sleep habits to improve sleep quantity and quality. Special strategies should be developed for shift workers and other professionals whose work schedules impose limitations on their sleep patterns.28
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