Ever since REM sleep (rapid-eye movement sleep) was discovered in the 1950s, scientists have wondered what its purpose was.
For example, recently both J. Allan Hobson and W.R. Klemm have suggested that REM sleep represents a halfway point between deep sleep and waking consciousness and helps us to transition successfully between those two states.
Jie Zhang has theorized that we create long term memories during REM sleep.
A new study has shown that REM sleep helps us to calm down after we have had stressful experiences. It does this by reducing the effect of activity in the amygdala, the part of the brain that is associated with processing emotions.
Post Traumatic Stress Disorder and REM Sleep
A number of years ago, combat veterans with post-traumatic stress disorder (PTSD) who took a drug for high blood pressure, known as Prazosin, began reporting that the drug seemed to stop their traumatic nightmares.
In 2003, a clinical study was performed in order to verify these claims.
10 veterans who had experienced combat in Vietnam and suffered from PTSD were studied. All of these veterans suffered from nightmares that were related to their traumatic experiences. The study showed that the veterans who were given Prazosin experienced a significant decrease in traumatic nightmares compared to the veterans who were given a placebo.
A new study of people who suffered from PTSD because of civilian trauma was performed in 2008. The subjects of this study were 13 people who had experienced severe traumas that included child sexual abuse, child physical abuse, rape, assault as an adult and being involved in a life-threatening car accident.
The results of this study also showed that Prazosin reduced the amount of trauma-related nightmares.
In both of these studies, subjects who took Prazosin experienced an increased amount of REM sleep.
Most people experience a reduction the production of epinephrine (adrenaline) and norepinephrine (noradrenaline) - hormones that are associated with stress - during REM sleep.
People who suffer from PTSD don't have normal REM sleep. Their REM sleep is disrupted, and they don't experience this reduction in stress hormones when they sleep.
By normalizing REM sleep, Prazosin helped to stop PTSD sufferers' traumatic nightmares.
Role of the Amygdala
The amygdala is associated with strong emotional responses. It is often associated with negative emotions, such as fear and anger.
In 2011, Els van der Helm, Justin Yao, Shubir Dutt and their colleagues performed an experiment in which they attempted to prove that REM sleep reduces the effect of activity in the amygdala, and this is what causes the reduction in stress hormones associated with REM sleep. They attempted to show that the calming effect of REM sleep causes a reduction in emotional reactions the following day.
The results of this study were published in the December 6, 2011 issue of Current Biology.
People who suffer from anxiety disorders such as PTSD constantly produce high frequency gamma brainwaves, which tend to be between 30 and 40 Hz, during REM sleep. These high frequency waves are associated with a high level of activity in the amygdala, high levels of epinephrine and norepinephrine and a state of hyperarousal - a condition of overwhelming stress and constant, extreme vigilance.
Van der Helm and her colleagues studied 34 healthy adults, between the ages of 18 and 30. They all had fMRIs (functional magnetic resonance image scans) to examine and record their brain activity.
During their fMRIs, each subject was told to look at 150 pictures that were designed to create an emotional response. They had to rate the pictures according to how much the pictures affected them emotionally, with 1 being the lowest rating and 5 being the highest.
12 hours later, each of the subjects had to take another fMRI and rate the same pictures again.
However, 18 of the subjects had a night of sleep between the two fMRIs, while the other 16 subjects spent a waking day in between the two scans.
An additional test, with a new set of pictures, was done during the second fMRI session, in order to ensure that the time of day wasn't affecting the results.
The study showed that the group that had a night's sleep between the two sessions saw a reduction in the number of intense emotional responses and a reduction in activity in the amygdala. They also experienced an increase in activity in the ventromedial prefrontal cortex, a part of the brain that is believed to regulate the emotions.
This group also experienced a reduction in gamma wave activity (as measured by an EEG) produced during REM sleep. There were significant correlations between the reduction in gamma wave activity, the reduction in amygdala activity and the decrease in the number of intense emotional reactions to the pictures.
This correlation applied only to REM sleep. It did not occur during NREM (non-rapid eye movement) sleep.
Among the group that stayed awake between fMRIs, intense emotional responses actually increased when looking at the pictures a second time.
Dreams and Negative Emotions
Earlier studies have shown that the amygdala is very active in REM sleep.
This can explain why, in people with normal sleep patterns who do not suffer from PTSD or other anxiety or mood disorders, dreams (which tend to be associated with REM sleep) are very likely to be associated with negative emotions.
Quantitative analyses of dream content have shown that the vast majority of dreams involve negative emotions.
The feeling of terror and the frightening hallucinations that are associated with sleep paralysis are ascribed to activity in the amygdala. Sleep paralysis is a state in which aspects of REM sleep are mixed with aspects of waking consciousness.
Previous studies have shown that the activity of the amygdala during REM sleep helps us to process emotional stimuli.
During REM sleep, the emotions that we experience in our waking lives are transferred to dreams, where they reveal themselves in unusual, and often bizarre, contexts.
It is up to a dream interpreter to determine how the emotions in our dreams relate to the emotions we experience in our waking lives. Sometimes it can be very difficult to recognize the emotional connection between a dream and a waking life event.
In people with PTSD, emotions are not filtered through dreams in this way. People with PTSD have dreams in which the traumatic event that they experienced happens just as it did in real life. Their negative emotions are not hidden in fantastic dreams that require interpretation.
In general, for psychologically healthy people, NREM dreams tend to be direct representations of events in the waking life of the dreamer, while REM dreams are likely to have a large amount of content that cannot be immediately recognized as part of the dreamer's waking life.
Van der Helm and her colleagues have suggested that the calming effect of REM sleep may be related to the way that REM sleep helps us to process our emotional memories.
REM sleep may help to make a stimulus seem less "new" - which leads to a decreased emotional response the following day.