By Laurie Scarborough
Nightmares, as defined in the Diagnostic and Statistical Manual of Mental Disorders, are distressing dreams that involve themes of fear, anxiety, escape or avoidance of threatening situations, and awaken the sleeper (American Psychiatric Association, 2013). Nightmares have a prevalence of 1-6% among adults, making them equally as ubiquitous as mood and anxiety disorders (APA, 2013), and thus worthy of consideration. Because of their distressing effects, researchers have speculated about their function, and new theories about nightmares are constantly being proposed (Windt, 2016). Because nightmares feel real during the dream (APA, 2013), many researchers believe that this means that nightmares serve a purpose for our waking functioning (Desseilles et al., 2010; Levin & Nielsen, 2009). Others believe that nightmares represent disordered behaviour that should be treated like any other mental disorder. Evolutionary, emotion-processing and fear extinction models have been postulated, which are summarised in the following discussion.
The leading evolutionary model for the function of nightmares is the threat simulation (TS) theory. It is postulated that nightmares are virtual rehearsals for threaten situations which allow the dreamer to build adaptive responses to threats and coping mechanisms for anxiety-provoking situations in order to prepare for such occurrences in reality, increasing the chance of survival under threatening conditions (Revonsuo, 2000; Windt, 2016).
TS has received much attention, however there are two objections to this theory. The first is forgetting dreams and nightmares (Desseilles et al., 2010; Windt, 2016); if nightmares are meant to enhance coping under threatening conditions, how can they be helpful if the dreamer cannot recall how they avoided a similar threat during a nightmare? Revonsuo argues that these TSs are encoded into implicit memory and are thus not required to be recalled explicitly. This explanation is hard to test however, as you cannot reproduce a threatening situation to test a participant’s coping if they cannot recall which threats they have rehearsed during nightmares. A second objection to TS is unrealistic threats in nightmares. TS assumes that nightmares are realistic, otherwise rehearsal would be pointless. Only 8-11% of nightmares feature realistic threats (Malcolm-Smith & Solms, 2004; Nielson & Levin, 2007), and only 3% of dreamers actually escape the threat in their nightmare (Malcolm-Smith & Solms, 2004). Thus, nightmares do not seem to effectively prepare the dreamer for genuine threats and it seems unlikely that nightmares function in this way.
The two notable emotion-related theories of nightmares are emotional-regulation and wish fulfilment models. Waking events and anxieties often influence dream content (Desseilles et al., 2010; Revonsuo, 2000; Solms & Turnbull, 2002) and waking emotional state also affects the quality of dreams; frightening dreams are more likely to occur when someone is stressed or emotionally overwhelmed (Levin & Nielsen, 2009; Perlis & Nielsen, 1993). Consequently, many dream theorists believe that dreams are a place to play out scenarios in order to process emotional responses to day-time events. Emotional regulation is thought to happen through problem solving during the dream, and learning to control emotional reactions (Kramer, 1991). The neurophysiology of rapid-eye movement (REM) sleep supports this theory. Limbic and paralimbic areas of the brain and the thalamus (which are all involved in emotional encoding, processing and expression) and the medial prefrontal cortex (which controls spontaneous emotional expression) are activated during REM sleep (Desseilles et al., 2010; Levin & Nielsen, 2009; Windt, 2016). This may explain why REM dreams are often so emotionally involved (Windt, 2016). Furthermore, the hippocampal anterior cingulate cortex, responsible for emotional memory consolidation is also activated during REM sleep (Desseilles et al., 2010). Since about 80-95% of dreams occur during REM sleep (Solms & Turnball, 2002), activation in these areas supports an emotional processing theory of dreams. But what of nightmares?
Under this model, nightmares are seen either as an extension of the emotional processing function of dreams, occurring when the dreamer reaches capacity for the emotional content contained in the dream (Kramer, 1991), or they are seen as a dysfunction of dreaming, since nightmares waken the dreamer before processing can complete (Nielsen & Levin, 2007). If the former were true, nightmares should help the dreamer to process dysphoric emotional content and thus affect after dreaming should be improved. Indeed, mood is changed by dreaming (Kramer, 1991), but findings disagree on the nature of this change. Roberts, Lennings and Heard (2009) found that nightmares are associated with lower depression scores, while people who do not experience nightmares have more likely to have high depression scores, indicating that nightmares do help process dysphoric emotions. Nielsen and Levin (2007) also found that nightmares are associated with long term depression remission. However, nightmares are associated with higher levels of anxiety, with anxiety being worse after sleep containing a nightmare compared to anxiety before sleep (Roberts et al., 2009). Nightmares are also associated with problems in regulating and expressing negative emotions (Nielsen & Levin, 2007), which indicates that nightmares do not help with emotional regulation. Thus a more convincing argument, is that nightmares are a dysfunction of the emotional processes of normal dreams.
Another emotional regulation theory is that because of the activation of emotional processing areas in the brain during REM sleep, an emotional surge occurs during REM sleep (Kramer, 1991; Nielsen & Levin, 2007). This surge of emotion is equalised by dreaming with emotional content, which is very common in REM dreaming (Revonsuo, 2000), in order to preserve sleep. Evidence supports this theory; people who dream sleep better than non-dreamers (Solms & Turnball, 2002). Nightmares are therefore a failure of this function of dreaming, as they result in the dreamer being woken up (Kramer, 1991; Nielsen & Levin, 2007; Solms & Turnball, 2002).
The final emotional regulation theory is that nightmares serve a wish fulfilment function. Suppressed wishes are simulated in dreams in order to regulate emotions toward these wishes and free guilt in consciousness over these wishes (Perlis & Nielsen, 1993; Roberts et al., 2009). Because we cannot indulge every desire in the waking world, our unconscious gratifies these desires in virtual reality (Solms & Turnball, 2002). Nightmares are then a masochistic wish fulfilment (Nielsen & Levin, 2007). Although originally a psychodynamic theory from Freud, and later Jung, the theory has recently garnered neurological support. The mesolimbic dopaminergic system (ML-DA), responsible for reward seeking and memory processes, is active during REM sleep (Perogamvros & Schwartz, 2012). This system is the neural correlate of Panksepp’s SEEKING, one of the basic emotions, described as “curiosity-interest-expectancy” (Perogamvros & Schwartz, 2012, p. 1936). REM sleep can cause SEEKING system activation, which triggers dream production (Solms & Turnball, 2002). Dreams are a “reward activation” (Perogamvros & Schwartz, 2012, p. 1943), and thus a wish fulfilment mechanism. Dreaming stops when the SEEKING pathway is damaged (Solms & Turnball, 2002), offering further evidence of its involvement in dreaming. Nightmares are a representation of the opposite of rewards: punishment or the punisher (Perogamvros & Schwartz, 2012), with a masochistic wish fulfilment function. The validity of this theory is difficult to assess since Freudian wishes or urges are unconscious, and thus not easily testable.
The final prominent theory of nightmares, is that they function as fear extinction mechanisms. During REM sleep, brain regions involved in fear conditioning, such as the amygdala and hippocampus (Desseilles et al., 2010), are activated, and it is through the frequent exposure to fearful stimuli that fears are reconditioned into non-fearful stimuli (Levin & Nielsen, 2009). This reconditioning is achieved through pairing fearful stimuli with muscle atonia and paralysis particular to REM sleep (APA, 2013; Solms & Turnbull, 2002), as well as the relaxation of sleep, similar to systematic desensitisation therapy (Perlis & Nielsen, 1993). By removing the fearful response to stimuli, it is thought that fears are eliminated (Nielsen & Levin, 2007). Frequent exposure to fearful stimuli is also meant to operate in a similar way to exposure therapy (Perogamvros & Schwartz, 2012), making the stimuli less and less stressful after each exposure (Roberts et al., 2009). Furthermore, the rapid eye movements that characterise REM sleep are also thought to desensitise the fearful stimuli, similar to eye movement desensitisation therapy (Perlis & Nielson, 1993).
This is a puzzling theory, since the fear extinction mechanism requires a fearful dysphoric dream in order to recondition a fear, which would take the form of a nightmare. However, nightmares awaken the dreamer and thus the fear extinction cannot complete (Perlis & Nielsen, 1993). Nightmare would seemingly be a dysfunction of ‘bad dreams’, or dysphoric dreams that do not awaken the dreamer (Levin & Nielsen, 2009).
There are many dream theories, however they remain speculative (Levin & Nielsen, 2009), since dreaming is difficult to study objectively (Solms & Turnball, 2002). Based on the theories discussed above, more evidence suggests that nightmares are better understood as a dysfunction or failure of the normal dreaming process, and should be treated as such. It also possible that dreams and nightmares are simply the normal accidental result of activation of certain brain areas during REM (Desseilles et al., 2010), however this does not mean that they are not subjectively meaningful to each person (Windt, 2016).
Dreams in general, and thus nightmares too, are most common during REM sleep (APA, 2013; Desseilles et al., 2010; Levin & Nielsen, 2009), and so much of the current research on dreaming and nightmares focuses only on REM dreaming. However, nightmares also occur during non-REM sleep and are qualitatively indistinguishable from REM dreams (Solms & Turball, 2002; Windt, 2016). This puts into question the validity of current nightmare and dream theories, since many of them explain dreaming and nightmares in terms of REM brain activation. Future research could focus on non-REM dreaming.
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