Motivation and emotion/Book/2010/Sleep and emotion

From Wikiversity
Jump to navigation Jump to search
Sleep and emotion
Epiphany-bookmarks.svg This page is part of the Motivation and emotion textbook. See also: Guidelines.

Overview[edit | edit source]

Sleeping Statue - - 50412.jpg
Rafael Nadal holding the 2008 Rogers Cup trophy2.jpg

Commonly coined by many parents throughout the years, the phrase; "get a good night's sleep and you'll feel better in the morning", may hold more truth to it than most parents probably ever imagined.

This chapter explores the relationship between sleep and human emotions and examines the interactive functions of sleep and sleep loss on human affective brain functioning and its expression.

What is emotion?[edit | edit source]

Emotion is the embodiment of physiological arousal (biological component), conscious experience (cognitive component) and overt expression (behavioural component).

Emotions are a multidimensional, psychological phenomenon, that exist as an essential and pervasive aspect of human experience. While possessing the capacity to colour and enrich our lives, emotions can also influence our perception, judgment and performance, making them powerful constructs that have the potential to either positively or negatively influence our behaviour and life experiences. Defined as the embodiment of physiological arousal (biological component), conscious experience (cognitive component) and overt expression (behavioural component), all of these elements combine to create a response to a precipitating situation or event (Myers, 2007).

Similar to the three primary colours, that in combination, can produce a range of alternative shades, research suggests that humans possess a set of innate, core emotions that can subsequently be blended and expressed. Ekman (1983) suggests that surprise, happiness, anger, fear, disgust and sadness are universal emotions that all humans share, and although other theorists have suggested alternative combination's of core emotions (Tomkins, 1962; Le Doux, 1996), it is now widely accepted that humans do possess as set of innate emotions that have the potential to evoke a wide range of reactions and subjective experiences.

But what actually causes emotion? Currently the three most widely accepted theories focus on either a biological or cognitive perspective, or the combination of both. The cognitive perspective centers around the idea that emotions are formed as a result of the individual's cognitive interpretation of a situation, that ultimately generates an emotional experience (Reeve, 2009). Lazarus (1984) suggests that it is not so much the event itself that elicits an emotional response, but the individual's appraisal of it. Similarly, Weiner (1986, as cited in Reeve, 2009), suggests that it is the way we interpret and reflect on life's events (successes or failures) that creates an emotional experience. As Lazarus puts it, emotions "cannot be understood solely in terms of what happens in the person or the brain, but grows out of ongoing transactions with the environment that are evaluated" (Lazarus, 1984, p.124).

Alternatively the biological perspective maintains that emotional processing is largely an automatic, unconscious process (Ekman, 1983) that is mediated by the brain through its neural pathways. Unlike the cognitive perspective, emotions are said to be formed prior to conscious awareness and are innate rather than learnt (Reeve, 2009). However other theorists maintain that emotions are created from a combination of both cognition and biology. Known as the Two-Systems View (Levenson, 1994 as cited in Reeve, 2009), this theory maintains that as a result of social and cultural learning, in combination with evolutionary and physiological pathways, individuals interpret and react to stimuli in the environment which subsequently evokes emotional reactions.

So which theory is correct? No one can be 100% sure, but either way it appears evident that emotions involve a complex chain of reactions that have the capacity to be influenced by other complex processes such as sleep.

What is sleep?[edit | edit source]

Sleep is an altered state of consciousness that involves the complex and active interaction of biological processes, that combine to aid in the feeling of overall health and well-being (Myers, 2007).

Sleep cycles through the night. As the diagram illustrates deep sleep occurs earlier in the night, while REM sleep (marked in red) is more dominant towards morning.

Falling into two main categories, sleep involves alternating patterns and progressive stages of non-rapid eye movement (NREM) and REM sleep. Studies indicate that these two stages cycle throughout the night, the four sub-stages of NREM sleep increasing sleep depth (Walker & van der Helm, 2009), while the ratio of NREM and REM sleep alternates throughout the night in 90 minute cycles (Walker, 2009). Whilst passing through these stages, changes in neurochemistry occurs (Saper, Chou, & Scammell, 2001). Subcortical cholmergic systems in the brainstem and forebrain become less active during NREM sleep (Lydic & Baghdoyan, 1988), as well as the firing rates of serotonergic raphe neurons, compared to levels when awake (Aston-Jones & Bloom, 1981). Similarly during REM sleep aminergic levels decrease, the brain subsequently being overshadowed by acetylcholine (Kametani & Kawamura, 1990).

In addition, during REM and NREM sleep, distinct differences in functional anatomy take place. During NREM sleep activity levels in the brainstem, basil ganglia and prefrontal and temporal regions of the brain dissipate, while activity in the hippocampus, amygdala and the anterior cingulate cortex elevate during REM sleep (Nofzinger, 2005).

Outside of the sleep cycle, the body's circadian system has also been found to adjust to the body's physiology and behaviour in preparation for sleep, regulating wakefulness over a 24-hour period (Foster & Wulff, 2005). Coupled then with the continuity, timing and sequence of the sleep stages, it appears evident that the architecture of sleep is a heterogenous system that supports and alters numerous brain functions that ultimately combine to regulate and promote a state of mental and physical homeostasis.

The Relationship between Sleep and Emotion - An Overview[edit | edit source]

In recent years considerable advances in cognitive neuroscience has seen a substantial amount of research conducted into the physiology of sleep and the functional impact that it has on neurocognitive processes (van der Helm, Gujar, & Walker, 2010; Chee & Chuah, 2008; Walker & Stickgold, 2006). However despite this progress little attention has been given to the interaction between sleep and affective brain functioning, and why an intimate relationship exists between sleep and clinical mood disorders.

Widely associated with negative affective states, such as depression, anger, fatigue and behavioural volitation (Lavidor, Weller, & Babkoff, 2003), sleep deprivation counteracts the restorative properties of sleep, that assist the body and mind to relax, reducing cognitive and physiological fatigue, and the subsequent development of negative affective states. Supported by several studies, sleep loss appears to effect the brain's ability to regulate emotion (Walker, 2009) and has been associated with a reduction in positive emotions following pleasing events and an increase in negative emotions following undesirable events (Zohar, Tzischinsky, Epstein, & Lavie, 2005). Similarly, Sonnentag, Binnewies, and Mojza (2008) found that sleep quality and quantity was a strong predictor of affective states, where one night of sleep deprivation corresponded with negative affect the following day. These findings suggest that the physiological processes that occur during sleep somehow effect individual's ability to recognise and regulate our emotions and imply that a causal relationship exists between sleep and emotional functioning.

The Role of Sleep in Emotion[edit | edit source]

Despite numerous studies spotlighting the interactive relationship between sleep and cognition, relatively little research has been conducted in the area of sleep and the impact that it has on emotional brain functioning and its affective expression. Nevertheless, the emergence of more recent studies (Walker, 2009; Walker & van der Helm, 2009) suggest that a multidimensional relationship exists between the various properties of sleep and the way in which we perceive, interpret and process emotional information.

Subsequently, this section aims to explore these relationships, in the hope of gaining a better understanding of the modulating effect that sleep plays on affective networks and human emotional experiences.

Emotional Regulation[edit | edit source]

It is estimated that humans spend approximately one third of their life sleeping (Kosslyn & Rosenberg, 2004). During this time the brain remains active as the rate of firing of various neurotransmitters and hormones fluctuate throughout the various stages of sleep, restoring the body and mind in preparation for another day of daily functioning (Kosslyn & Rosenberg). However despite substantial research into the area of sleep and cognition, relatively little is known about the impact that sleep has on our overall ability to regulate our emotions.

For most people a lack of sleep is commonly associated with irritability and affective volitation, although little thought is often given to the physiological reasons behind this, and how the many functions of sleep interact to modulate emotional stability. Therefore in considering the impact that sleep has on human emotion, attention also needs to be given to the way we process and regulate the vast amount of emotions that we express and feel.

The facial muscles of the face that assist in emotional expression

Primarily expressed via our faces, facial expression helps us to instigate and maintain relationships and portray positive and negative reinforcement for elicited behaviours (Ekman, 1983). Supported by the Facial Feedback Hypothesis (Tomkins, 1962 as cited in Reeve, 2009), it suggests that emotions are biologically based and occur in response to nerve impulses from our facial behaviour, that include changes in facial temperature and the muscular and glandular responses of the face.

The face activates different emotions as a result of external and internal events that subsequently increases neural activity throughout areas of the brain, resulting in emotional activation via the face and emotionally driven cognitive and physical responses (Ekman, 1983).

In support of the Facial Feedback Hypothesis, empirical evidence has started to emerge, suggesting that sleep deprivation impacts on neural, physiological and subjective aspects of emotions (Walker, 2009). In a study by Yoo, Gugar, Hu, Jolesz, and Walker (2007), functional MRI (fMRI) images found that emotional brain reactivity in the amygdala was amplified following one night of sleep deprivation. Crucial in emotional processing, the amygdala was found to increase in volume by 300% when expose to adverse stimuli, while activation in the medial prefrontal cortex and in particular the anterior cingulate cortex (ACC), decreased. See this link for a fMRI which depicts the amygdala following one night of sleep deprivation. Results also found that compared to the control group, the participants deprived of sleep showed reduced functional connectivity between the amygdala and the medial prefrontal cortex; an area know to regulate the amygdala (Walker, 2009). Likewise, following sleep loss, negative mood states were also amplified, as well as reactivity to negative stimuli. Similarly, Dinges et al. (1997) found that emotional disturbance and emotional reactivity over a one week period progressively increased, based on results from a Questionnaire Mood Scale and the daily journals of the participants.

The amygdala is found in the subcortyical region of the brain and is thought to be important in the processing of emotional aspects of experience and emotional memory (Goldstein, 2008).

In sum, these studies indicate that an intrinsic relationship exists between sleep and the regulation of emotional brain functioning, and offer a tangible, physiological explanation as to why sleep loss results in negative affective states.

Emotional Processing and Recognition[edit | edit source]

Supporting the findings that sleep deprivation impacts on emotional regulation, research also suggests that our ability to process and recognise emotions is also impaired following sleep loss. Argued to be one of the most meaningful ways of conveying emotion, facial expressions provide visual cues to the outside world that allows us to not only express ourselves, but recognise and interpret the emotions and moods of those around us.

Highlighting the important role that emotional recognition plays, people with Autism, who lack the ability to detect, process and therefore be guided by facial expressions, live with impaired social skills and an inability to interact with others on an emotionally meaningful level (Sasson et al., 2007).

Emotional Expressions

Subsequently, recent findings that indicate that sleep loss negatively impacts on emotional facial recognition suggests potentially serious social implications. In a study by van der Helm, Gujar, and Walker (2010), participants were asked to view gradiently morphed versions of Ekman's pictures of facial affect, which ranged from neutral to increasingly emotional (Happy, Sad and Angry), following either 30 hours of sleep deprivation or complete rest. The results found that sleep loss disrupted the participant's ability to accurately recognise subtle emotional intensities and were particularly pronounced in 'Happy' and 'Angry' expressions. Interestingly, this impairment was particularly marked in women, suggesting a gender specific vulnerability to sleep loss. Complementing these findings, Pallesen et al. (2004) found that after only one night of sleep deprivation the speed and accuracy at which participants could rate emotional facial expressions declined rapidly. Similarly, specific REM sleep deprivation was found to reduce participants' reactivity to previously viewed adverse emotional stimuli.

Coupled then with the findings that sleep loss also adversely effects emotional regulation, it would appear evident that a sleep deprived brain interferes with aspects of emotional processing and recognition at a functional level.

Memory Processing and Consolidation[edit | edit source]

Similar to the role it plays in emotional recognition and regulation, sleep has been implicated in emotional memory processing, a process that involves two main stages: the encoding of new information and memory consolidation which occurs post learning (Walker, 2009).

Affective Memory Encoding[edit | edit source]

The process of memory encoding and formation has been found to be largely influenced by the emotional essence of the stimuli at the time of learning (Phelps, 2004), as indicated by studies suggesting that emotionally arousing information is better remembered than neutral information (Buchanan & Lovallo, 2001; Heuer & Reisberg, 1990). Thought to be predominantly facilitated by the amygdala and structures within the medial temporal lobe (including the hippocampus), fMRI studies have shown that the right amygdala is more strongly activated when viewing emotionally arousing stimuli (McGaugh, 2004). Subsequent studies have also suggested that together, the amygdala and the anterior hippocampus are instrumental in the encoding of emotional memories, the co-activation for encoding not only influenced by the emotional content of the information, but by how well the brain is sleep rested (Walker & van der Helm, 2009). Walker (2009) found that when sleep deprived, participants showed a 40% reduction in their ability to encode new memories, an effect that was particularly pronounced in the encoding and subsequent retention of positive emotional stimuli. However little effect was found for negative stimuli, a result that could be clinically significant given the dyadic relationship that exists between mood disorders and sleep disturbances. Additionally, other studies have also indicated that memory encoding is more pronounced when the stimuli is emotionally salient (Anderson, Wais, & Gabrieli, 2006) and when sleep precedes the event (Maquet, 2001).

Affective Memory Consolidation[edit | edit source]

The role of sleep in emotional memory consolidation has also been demonstrated. Sleep loss disrupts the consolidation process and impairs recollection, an effect particularly pronounced following REM sleep deprivation (Walker & Stickgold, 2004). Research suggests that the cognitive processes that occur during sleep, allow the brain to move from a state of encoding to that of memory consolidation (Rasch, Burn, & Gais, 2006). In a study by Gais, Phihal, Wagner, and Born (2000), participants were woken at different stages during the sleep cycle. The results indicated that during slow wave sleep (SWS), stages 3 and 4 of NREM sleep, memory consolidation was initiated and that during REM sleep the consolidation process was reinforced. Furthermore, in a study by Hu, Stylos-Allen, and Walker (2006), the consolidation of emotionally arousing pictures compared to neutral ones, were contrasted following a 12-hour period of sleep deprivation. Participants in the control group (no sleep loss) showed greater emotional memory retention, these results adding further weight to the findings of Born and Wagner (2004), who found that the recall of emotional memory tasks significantly improved when followed by sleep.

Collectively, this research suggests that sleep is instrumental in the encoding, consolidation and recollection of affective memories. The impact of sleep deprivation has been consistently associated with the emotional significance of the material being processed to memory and has been linked to the engagement of different cortical areas of the brain that are associated with emotional states.

Dreaming[edit | edit source]

"For what one has dwelt on by day, these things are seen in visions of the night" Menander of Athens (342-292 B.C.), Fragments

Dreams are a combination of emotions, images and thoughts that cycle throughout REM and NREM sleep.

Strengthening the position that sleep is fundamentally vital in emotional processing and regulation, is the crucial role that dreams play during sleep. Occurring during both REM and NREM sleep stages, although more memorable if woken during REM sleep (Kosslyn & Rosenberg, 2004), research suggests that dreaming aids in the relief of emotional trauma. In a study by Cartwright, Luten, Young, Mercer, and Bear (1998), during REM sleep the process and content of dreams were found to help in the resolution of emotional conflict, concluding that sleep which incorporated dreaming, assisted individuals to resolve emotional trauma and resulted in more positive mood states the following day. Similarly in a study by Hobson, Pace-Schott, and Stickgold (2000), 75% of dreams that occurred during REM sleep were found to have emotional content. Fosse, Fosse, Hobson, and Sickgold (2003) also linked dream content to prevailing emotional concerns.

Initially proposed by Sigmund Freud, who maintained that dreams allowed the individual to satisfy their unconscious desires and express inner conflict (Kosslyn & Rosenberg, 2004), the idea that dreams are linked to emotional states may therefore be not so far from the truth. Such findings add clinical strength and validity to the strong link that already exists between mood disorders and sleep disturbance, strengthening the position that sleep is intrinsically linked to emotional health and well-being.

The Effect of Sleep Deprivation on Affective States[edit | edit source]

Mood Disorders comprise of a group of disorders that involve severe and enduring disturbances in emotionality, ranging from severe depression to euphoria (based on DSM-IV-TR, 2000).

Sleep disturbance, that can comprise of insomnia, restless or broken sleep has been consistently associated with impaired cognitive performance, physiological fatique and reduced feelings of overall health and well-being (Papadimitrious & Linkowski, 2005). Likewise many people suffering from affective disorders complain about disruptive sleeping patterns - the DSM-IV-TR (2000) consistently lists sleep disturbance as a formal symptom, or common feature, across most psychiatric mood disorders. Sleep deprivation is now commonly viewed as an antecedent to mental health conditions, although recent findings suggest that their co-occurrence is more than epiphenomenal (Papadimitrious & Linkowski, 2005). For additional information on the impact of sleep deprivation and its impact on mood disorders see this link, which outlines the Sleep to Forget and Sleep to Remember (SFSR) Hypotheses (pp, 741-743).

The next section discusses two of the most prevalent psychiatric disorders, major depression and anxiety, in relation to sleep, as well as the possible bi-directional consequences of long term sleep loss.

Sleep Disturbances and Depression[edit | edit source]

Major Depression is marked by consistent (2 weeks plus) levels of sadness, low energy, insomnia, low self-worth and guilt (based on DSM-IV-TR, 2000).

Found to be an increasingly prevalent mental health concern in today's society (Adewuya, Ola, & Aloba, 2007), major depression has been consistently linked to sleep abnormalities (DSM-IV; American Psychiatric Association, 2000). Subsequently associated with the development and maintenance of the disorder (Harvey, 2001), increased patterns of sleep latency and irregular patterns of REM sleep, including abnormally short REM sleep latency have been found to be a hallmark feature of major depression (Walker & van der Helm, 2009). In a study by Armitage (2007, as cited in Walker & van der Helm, 2009), depression was linked to a faster entry into REM sleep and a longer first period of REM sleep, resulting in a higher percentage of REM sleep throughout the night. Consequently, the increased amount of REM sleep suggests that limbic structures may become over activated; Norfzinger (2005) concluding that as a result, depressed people may experience and encode negative stimuli more intensely, ultimately resulting in the maintenance of the condition. Similarly, the recommencement of normal sleeping patterns have been found to reduce the risk of relapse into depression and have been associated with positive responses to anti-depressant medication (Walker & van der Helm, 2009). In a study by Skouteris, Germano, Wertheim, Paxton, & Milgrom (2008), sleep quality during pregnancy was examined in relation to depression. Results indicated that poor quality sleep during the first trimester of pregnancy significantly increased the likelihood of depression late in pregnancy.

Collectively, these findings suggest that sleep quality may be an antecedent to depression, that is then subsequently maintained due to the ongoing consequences of sleep deprivation.

Sleep Disturbance and Anxiety Disorders[edit | edit source]

Yakunchikova Fear

Exhibiting similar characteristics to major depression, a positive correlation has also been found to exist between sleep disturbances and anxiety disorders. Estimated as having a lifetime prevalence of approximately 20% (Kessler et al., 1994), sufferers of anxiety disorder report difficulties in initiating and maintaining sleep (Papadimitriou & Linkowski, 2005). In a study by Ohayon and Roth (2003), 43% of the participants in the study disclosed that anxiety appeared prior to any occurrences of sleep disturbances, 18% said that sleep disturbances appeared first and 39% reporting a co-occurrence.

During sleep, polysomnographic studies have revealed that, similar to those suffering from major depression, longer sleep latency was detected (Papadimitriou & Linkowski, 2005). However unlike major depression, individuals with anxiety disorders measured no significant changes in REM sleep, while NREM time was reduced (Benca, Obermeyer, Thisted, & Gillin, 1992).

Anxiety is a negative mood state marked by physical tension, apprehension and fears about the future (based on DSM-IV-TR, 2000).

However despite the differences in sleep stage cycles, there appears to be a significant causal link between sleep disturbances and both depression and anxiety. The sleep architecture in response to sleep deprivation in both depression and anxiety are different compared to normal individuals, with the pattern of REM sleep differentiating anxiety from major depression in relation to sleep patterns (Papadimitriou & Linkowski, 2005). While the causes of these differences are still yet widely unknown, it does highlight the functional role that sleep plays in the regulation and maintenance of positive affective states.

Chapter Summary[edit | edit source]

In a society where suboptimal sleep levels are now considered the norm, the long term impact of sleep loss on society is still yet to unfold. Research studies conclusively suggest that sleep deprivation has a negative impact on affective states and subjective experiences, with a causal link being found between emotional information processing, recognition and the various properties of sleep.

This chapter began by addressing "What constitutes an emotion"? Defined as the embodiment of physiological arousal, conscious experience and overt expression, it encapsulates both the cognitive and biological perspectives of emotional causality, suggesting that they both play a crucial role in the activation and regulation of emotion.

Like emotions, sleep is viewed as the complex interaction of biological and cognitive processes that have the ability to influence our health, well-being and affective states. Consisting of the complex interaction of five stages of sleep (REM and the four sub stages of NREM sleep), they alternate throughout the night, creating changes in neural chemistry that then interact with the body's circadian system to regulate and promote a state of mental and physical homeostasis.

Recent advances in cognitive neuroscience has created an opportunity to more thoroughly explore the functional relationship between sleep and emotion. Widely associated with negative affective states, recent studies have begun to explore the neural and physiological processes that occur during sleep and how they impact on the recognition, regulation and emotional memory processing in humans. The amygdala and specific areas in the medial prefrontal cortex of the brain, have been identified as playing a vital role in the maintenance and control of emotional processing, studys consistently indicating that sleep deprivation reduces the functional connectivity between the amygdala and the medial prefrontal cortex. fMRI images also suggest that sleep deprivation over stimulates the amygdala and deactivates the medial prefrontal cortex when exposed to emotional stimuli.

Sleep deprivation has also been shown to reduce our ability to process and recognise emotions in other people. Occurring primarily through facial expressions, extreme emotions such as Happy and Angry have been shown to be particularly susceptible as a result of sleep loss, indicating that a lack of sleep creates a functional vulnerability, with the potential to cause significant ramifications for society, given the vital role that facial expressions play in visual communication.

Similarly, sleep was shown to assist in the encoding and consolidation of emotional memories. Predominantly facilitated by the amygdala and areas of the medial temporal lobes, individuals when sleep deprived were found to reduce their capacity to encode new and emotionally positive memories by 40%. Studies also indicated that memory consolidation and recollection was impaired when the brain was deprived of sleep. Found to disrupt and alter the cycles of REM and NREM sleep, the sequence of information encoding, consolidation and recall, via neural pathways, deteriorated when sleep loss prevailed.

Finally the effects of sleep deprivation on affective states were discussed. Research found that the co-morbidity that exists between sleep disturbances and psychiatric conditions, such as major depression and anxiety disorders, may be more than epiphenomenal, but rather bi-directional. Evidence suggests that sleep disturbances, which have been shown to negatively impact on emotional memory processing, may not only facilitate the maintenance of mood disorders, but play a key role in their initiation. Furthermore, the role of dreaming during sleep was found to assist in the resolution of emotional conflict, strengthening the theory that sleep and its functional properties are an antecedent to mental health and well-being.

Subsequently, sleep deprivation which has become a ubiquitous part of today's society, is likely to become a systematic problem unless the value of sleep, beyond its role to physically restore the body, is realised.

Key Terms[edit | edit source]

Emotion: the embodiment of physiological arousal (biological component), conscious experience (cognitive component) and overt expression (behavioural component), which combine to create a response to a precipitating situation or event

Sleep: is an altered state of consciousness that involves the complex and active interaction of biological processes, that combine to aid in the feeling of overall health and welbeing

Amygdala: is found in the subcorical region of the brain and is thought to be important in the processing of emotional aspects of experience and emotional memory

Dreams: are a combination of emotions, images and thoughts that cycle throughout REM and NREM sleep

Mood disorders: comprise of a group of disorders that involve severe and enduring disturbances in emotionality, ranging from severe depression to euphoria

Major depression: is marked by consistent (2 weeks plus) levels of sadness, low energy, insomnia, low self-worth and guilt

Anxiety: is a negative mood state marked by physical tension, apprehension and fears about the future

See Also

References[edit | edit source]

Adewuya, A. O., Ola, B. A., & Aloba, O. O. (2007). Prevalence of depressive disorders and a validation of the Beck depression inventory among Nigerian adolescents. European Child and Adolescent Psychiatry, 16, 287-292. doi: 10.1007/s00787-006-0557-0

American Psychiatric Association. (2000). Diagnostic and statistical manual of mental disorders (4th Rev. ed.). Washington, DC: American Psychiatric Association.

Anderson, A. K., Wais, P. E., & Gabrieli, J. D. (2006). Emotion enhances remembrance of neutral events past. Proceedings of the National Academy of Sciences, USA, 103, 1599-1604.

Aston-Jones, G., & Bloom, F. E. (1981). Activity of norepinephrine containing locus coeruleus neurons in behaving rats anticipates fluctuations in the sleep-wake cycle. Journal of Neuroscience, 1, 876-886.

Benca, R. M., Obermeyer, W. H., Thisted, R. A., & Gillin, J. C. (1992). Sleep and psychiatric disorders. A meta-analysis. Archives of General Psychiatry, 49, 651-668.

Born, J., & Wagner, U. (2004). Awareness in memory: Being explicit about the role of sleep. Trends in Cognitive Science, 8, 242-244.

Buchanan, T. W., & Lovallo, W. R. (2001). Enhanced memory for emotional material following stress-level cortisol treatment in humans. Psychoneuroendocrinology, 26, 307-317.

Cartwright, R., Luten, A., Young, M., Mercer, P., & Bears, M. (1998). Role of REM sleep and dream affect in overnight mood regulation: A study of normal volunteers. Psychiatry Research, 81, 1-8.

Chee, M. W., & Chuah, L. Y. (2008). Functional neuroimaging insights into how sleep and sleep deprivation effects memory and cognition. Current Opinion in Neurology, 21, 417-423.

Dinges, D. F., Pack, F., Williams, K., Gillen, K. A., Powell, J. W., Ott, G. E., Aptuwicz, C., & Pack, A. I. (1997). Cumulative sleepiness, mood disturbance, and psychomotor vigilance performance decrements

during a week of sleep restricted to 4-5 hours per night. Sleep, 20, 267-277.

Ekman, P. (1983). Facial expression and emotion. American Psychology, 48, 384-392.

Fosse, M. J., Fosse, R., Hobson, J. A., & Stickgold, R. J. (2003). Dreaming and episodic memory: A functional dissociation? Journal of Cognitive Neuroscience, 15 (1), 1-9.

Foster, R. G., & Wulff, K. (2005). The rhythm of rest and excess. Neuroscience, 6, 407-414.

Gais, S., Phihal, W., Wagner, U., & Born, J. (2000). Early sleep triggers memory for early visual discrimination skills. Nature Neuroscience, 3, 1335-1339.

Goldstein, E. B. (2008). Cognitive Psychology. Belmont, USA: Wadsworth.

Harvey, A. G. (2001). Insomnia: Symptom or diagnosis? Clinical Psychological Review, 21, 1037-1059.

Heuer, F., & Reisberg, D. (1990). Vivid memories of emotional events: The accuracy of remembered minutiae. Memory and Cognition, 18, 496-506.

Hobson, J., Pace-Schott, E., & Stickgold, R. (2000). Dreaming and the brain: Towards a cognitive neuroscience of conscious states. Behavioural Brain Science, 23, 793-842.

Hu, P., Stykos-Allen, M., & Walker, M. P. (2006). Sleep facilitates consolidation of emotionally arousing declarative memory. Psychological Science, 17, 891-898.

Kametani, H., & Kawamura, H. (1990). Alterations in acetylcholine release in the rat hippocampus during sleep-wakefulness detected by intracerebral dialysis. Life Science, 47, 421-426.

Kessler, R. C., Mcgoagle, K. A., Zhoa, S., et al (1994). Lifetime and 12-month prevalence of DSM-III-R psychiatric disorders in the United States. Results from the National Comorbidity Survey. Archives of General Psychiatry, 51, 8-19.

Kosslyn, S. M., & Rosenberg, R. S. (2004). Psychology: The brain, the person, the world. (2nd ed.). Boston, Pearson.

Lavidor, M., Weller, A., & Babkoff, H. (2003). How sleep is related to fatigue. British Journal of Health Psychology, 8, 95-105.

Lazarus, R. S. (1984). On the primacy of cognition. American Psychologist, 39, 124-129.

LeDoux, J. E. (1996). The emotional brain: The mysterious underpinnings of emotional life. New York: Simon & Schuster.

Lydic, R., & Baghdoyan, H. A. (1988). Handbook of behavioural state control: Cellular and molecular mechanisms. Buca Raton, FL: CRC Press.

Maquet, P.(2001). The role of sleep in learning and memory. Science, 294, 1048-1052.

McGaugh, J. L. (2004). The amygdala modulates the consolidation of memories of emotionally arousing experiences. Annual Review of Neuroscience, 27, 1-28.

Myers, D. G. Psychology. (8th ed.). NY: Worth.

Nofzinger, E. A. (2005). Functional neuroimaging of sleep. Seminars in Neurology, 25, 9-18.

Ohayon, M. M., & Roth, T. (2003). Place of chronic insomnia in the course of depressive and anxiety disorders. Journal of Psychiatric Research, 37, 9-15.

Pallesen, S., Johnsen, B., Hansen, A., Thayer, J. F., Olsen, T., & Hugdahl, K. (2004). Sleep deprivation and hemispheric asymmetry for facial recognition reaction time and accuracy. Perceptual and Motor Skills, 98, 1305-1314.

Papadimitriou, G., & Linkowski, P. (2005). Sleep disturbance in anxiety disorders. International Review of Psychiatry, 17, 229-236.

Phelps, E. A. (2004). Human emotion and memory: Interactions of the amygdala and hippocampal complex. Current Opinion in Neurobiology, 14, 198-202.

Rasch, B., Born, J., & Gais, S. (2006). Combined blokeade of cholinergic receptors shifts the brain from stimulus encoding to memory consolidation. Journal of Cognitive Neuroscience, 18, 793-802.

Reeve, J. (2009). Understanding motivation and emotion (5th ed.). Hoboken, NY: Wiley.

Saper, C. B., Chou, T. C., & Scammell, T. E. (2001). The sleep switch: Hypothalamic control of sleep and wakefulness. Trends in Neuroscience, 24, 726-731.

Sasson, N., Tsuchiya, N., Hurley, R., Couture, S. M., Penn, D., Adolphs, R., & Piven, D. (2007). Orienting to social stimuli differentiates social cognitive impairment in autism and schizophrenia.

Neuropsychologia, 45, 2580-2588. doi: 10.1016/j.neuropsycholgia.2007.03.009

Skouteris, H., Germano, C., Wertheim, W. H., Paxton, S. J., & Milgrom, J. (2008). Sleep Research, 17, 217-220. doi: 10.1111/j.1365-2869.2008.00655.x

Sonnentag, S., Binnewies, C., & Mojza, E. J. (2009). "Did you have a nice evening?". A day-level study on recovery experiences, sleep and affect. Journal of Applied Psychology, 3, 674-684.

van der Helm, Els., Gujar, N., & Walker, M. P. (2010). Sleep deprivation impairs the accurate recognition of human emotions. Sleep, 33,, 335-342.

Walker, M. P. (2009). The role of sleep in cognition and emotion. Annuals of the New York Academy of Sciences, 1156, 168-197. doi: 10.1111/j.1749-6632.2009.04416.x

Walker, M. P., & Stickgold, R. (2004). Sleep-dependent learning and memory consolidation. Neurons, 44, 121-133.

Walker, M. P., & Stickgold, R. (2006). Sleep, memory and plasticity. Annual Review of Psychology, 10, 139-166.

Walker, M. P., & van der Helm, Els. (2009). Overnight therapy? The role of sleep in emotional brain processing. Psychological Bulletin, 135, 731-748. doi: 10.1037/a0016570

Yoo, S. S., Gujar, N., Hu, P., Jolesz, F. A., & Walker, M. P. (2007). The human emotional brain without sleep: A prefrontal amygdala disconnect. Current Biology, 17, R877-R878.

Zohar, D., Trischinsky, O., Epstein, R., & Lavie, P. (2005). The effects of sleep loss on medical residents' emotional reactions to work events: A cognitive-energy model. Sleep, 28, 47-54.