The emotional brain and sleep: An intimate relationship

Summary

Research findings confirm our own experiences in life where daytime events and especially emotionally stressful events have an impact on sleep quality and well-being. Obviously, daytime emotional stress may have a differentiated effect on sleep by influencing sleep physiology and dream patterns, dream content and the emotion within a dream, although its exact role is still unclear. Other effects that have been found are the exaggerated startle response, decreased dream recall and elevated awakening thresholds from rapid eye movement (REM)-sleep, increased or decreased latency to REM-sleep, increased REM-density, REM-sleep duration and the occurrence of arousals in sleep as a marker of sleep disruption. However, not only do daytime events affect sleep, also the quality and amount of sleep influences the way we react to these events and may be an important determinant in general well-being. Sleep seems restorative in daily functioning, whereas deprivation of sleep makes us more sensitive to emotional and stressful stimuli and events in particular. The way sleep impacts next day mood/emotion is thought to be affected particularly via REM-sleep, where we observe a hyperlimbic and hypoactive dorsolateral prefrontal functioning in combination with a normal functioning of the medial prefrontal cortex, probably adaptive in coping with the continuous stream of emotional events we experience.

Introduction

Even though the relationship between pre-sleep emotional experiences and quality of sleep, as well as the reason why we sleep, seems intuitively evident, until recently this topic has gotten increased attention. The scarce amount of research is surprising given the importance of sleep in emotional well-being and the occurrence of disturbed sleep in many psychological and psychiatric disorders. In this review, the scope will concentrate on the relationship between emotion and sleep, particularly rapid eye movement (REM)-sleep and its emotion modulatory and even emotion regulatory functions in especially healthy individuals.

In the first part, we review how to understand how emotional situations affect sleep and how sleep affects emotional processing of affective information. In the second part, the discussed findings are evaluated in light of neurophysiological insights.

Research findings appear to confirm our own life-experience where daytime events, especially emotionally stressful events, have an impact on the quality of our sleep and well-being. Daytime emotional stress may have a twofold effect on sleep: first by influencing sleep physiology and second by influencing dream patterns, dream content and the emotion within a dream, although its exact role still is unclear. Even social phenomena such as reduced social support and increased patterns of avoidance in a person's emotion regulation, appear to result in psychological distress and sleep complaints.¹ Furthermore, the association between cortical and emotional arousal and disrupted sleep in individuals with insomnia suggests a strong relationship between daytime events and disrupted sleep. As a matter of evidence, it is clear that the individual's response and coping with the emotional stress associated with daytime events involves the capacity to de-arouse or disengage from active wake processing interfering with the normal initiation of sleep processes. Sleep latency, especially REM-sleep alterations or abnormalities, have been related to variables associated with the affective state of individuals during the day.² Watching aversive films before sleep for instance, has been reported to influence emotional experiences in the first REM-periods of the night.³ Reported effects of emotion and of pre-sleep mood and stress on sleep include: decreased dream recall and elevated awakening thresholds from REM-sleep,⁴ increased or decreased latency to REM-sleep, increased REM-density, REM-sleep duration,⁵ occurrence of arousals in sleep as a marker of sleep disruption⁶ as well as disturbances in sleep continuity.

In some studies, investigating the effects of especially acute stress exposure on sleep in healthy persons found that REM-sleep alterations are more frequent than nREM-alterations. For instance, in a study of Germain and others,⁷ they found that acute stress exposure where subjects had been told that they had to give a speech in the morning and that their performance would be evaluated, resulted in an increase in REM-density across REM-periods, a decrease in late-night average REM-count and a slower rate of increase across successive REM-periods immediately after the stress exposure. The result that the average REM-density increased is in line with some previous studies on the effects of acute stress exposure.*8, 9 However, in other studies, acute stress exposure has been associated inconsistently with increased percentage of REM-density and REM-sleep duration*10, 11 or alterations within REM- or nREM-sleep. Hall and coworkers¹² correctly claim that the pathways in which stress affects sleep and produces frequent awakenings from sleep, lightens NREM-sleep, or affects the quantitative and qualitative components of REM-sleep are not as well defined. In the case of clinical disorders subsequent to the experience of a traumatic life event, such as posttraumatic stress disorder, also only a few studies investigated posttraumatic stress disorder directly after the traumatic life event. In one such study investigating three patients hospitalized for acute combat fatigue, sleep was fragmented, of short duration and characterized by high motoric reactivity. REM-sleep had been found to be rare and short.¹³

REM-sleep alterations also have been found in people undergoing transient or chronic life events with and without comorbid depression.⁹ With depression, a prolonged duration of the first REM-period, an increased density of eye movements, REM-percentage, and total sleep time have been reported.13, 14 Also, in suicide-attempters, an increase in REM-activity and REM-sleep duration over the entire night and in the first REM-sleep period have been found.¹⁵ These abnormalities in patients with depression in the first non-REM/REM-sleep cycle¹⁶ have been related to the dysfunctions in the emotional and cognitive processing in clinical disorders such as depression.¹⁷ Also, a reduced REM-sleep latency has been identified as an objective indicator of depressive disorder and suicide.¹⁸ Furthermore, increased risk for relapse in depressive persons and in alcoholics has been found to be related to increased REM-density.*8, 19 Furthermore, at pre- and post-treatment psychotherapy, affect intensity in depressed men has been correlated significantly and positively with phasic REM-sleep measures, characterized by REM-bursts.*17, 20 A decrease in REM-density, on the other hand, has been correlated with remission with therapy and reductions in negative affect intensity in depression.*8, *17, *21 Phasic REM-sleep decreases over the course of psychotherapeutic treatment in depressed patients, in comparison with more tonic aspects of REM-sleep involving REM-sleep latency, wherein change may be a marker of manifestations of depression.²¹ In correspondence, failure to remit with psychotherapy in depressive persons has been correlated with increased REM-density.14, *8

In summary, abundant evidence confirms a relationship between the emotional experiences we have during the day and changes in sleep physiology, in particular modified, enhanced or decreased REM-sleep.