When we dream, it is not just our imagination creating a virtual reality throughout the night for us to explore. While we may think dreaming does not serve an important purpose, it in fact holds multiple beneficial functions. One main function of dreaming that has been hypothesized is that of the Overnight Therapy Theory. Matthew Walker, author of Why We Sleep, proposed this theory as a function of dreaming, where dreams in REM sleep function as a way to help tend to our emotional and mental health (Walker 207). Walker states that in REM sleep dreaming, strong emotions are separated from emotional experiences that we go through in our waking hours, providing “emotional resolution when you awake the next morning” (Walker 207). Walker, among others, have studied the mechanisms behind this idea of stripping emotion from certain memories. One such mechanism relates to the stress chemical noradrenaline in the brain. When we enter dreams in REM sleep, the supply of noradrenaline is shut off, creating a stress-free environment in the brain to dream about the emotional experiences from the waking hours of your day. With this newly created stress-free environment, other mechanisms are also at play. The amygdala, which is an area of the brain related to emotion, as well as the hippocampus, an area of the brain related to memory, are both reactivated. With these two factors in mind, Walker wanted to find out if this REM-sleep dream state acts as a “soothing balm” (Walker 208).

            To answer this question, Walker came up with the overnight therapy theory mentioned above. In this theory, Walker predicted that the reason why we sleep is to remember certain details of an experience, but at the same time, we sleep to forget the strong emotions originally paired with these experiences. This is called the sleep to forget and sleep to remember hypothesis, where there is a “reduction in the affective tone associated with [experiences’] recall” (Walker & van der Helm, 2009). All of this, Walker predicted, occurs due to the REM-sleep dreaming stage. To study this new theory, Walker conducted an experiment where participants viewed emotional images in an MRI scanner to measure the emotional brain activity of each participant, as well as ratings of emotion the participants felt. The first group looked at the images once in the morning and then in the evening 12 hours later, staying awake between each session. The second group, however, looked at the images in the evening and then the next morning after a full night of sleep. Walker found that those in the second condition reported feeling less emotional during the second viewing of the images, which was mirrored in the decreased activity in the amygdala and increased activity in the prefrontal cortex. These results support the overnight therapy theory in that the dreaming state of REM sleep is what causes emotional decreases and separations from emotional experiences (Walker 209-210).

            Another study supporting the overnight therapy theory Walker mentions is one conducted by Rosalind Cartwright. In this experiment, Cartwright studied the dream content of people with depression caused by an emotional experience, specifically one of a breakup or divorce. Cartwright had participants record their dreams, and she looked for any emotional themes during follow-ups for a year to see any changes in depression and anxiety. Cartwright found that the participants who dreamt about their emotional experience had lower levels of depression after a year compared to those who dreamed, but not about their emotional experience. These findings support the idea that we need REM-sleep dreaming in order to separate emotions from experiences, but an additional result Cartwright found was that the dreams need to be content-specific in order for the separation to occur (Walker 211; Cartwright et al., 2006).

            The REM-sleep dreaming state is an important aspect of our sleep, and it is highly beneficial for us to achieve enough sleep during this state. However, some people are not able to achieve and maintain REM-sleep, such as those with sleep abnormalities or certain mental illnesses. Two of these mental illnesses are Post Traumatic Stress Disorder (PTSD) and Major Depressive Disorder (MDD). Patients with PTSD tend to experience the symptom of intrusive recurring nightmares and flashbacks about their traumatic experiences, with a dysregulation of REM sleep (Walker & van der Helm, 2009). Walker wondered whether the overnight therapy theory mechanisms mentioned above are damaged in patients with PTSD, as he predicted that those with PTSD have not successfully been able to separate their emotions from their traumatic memories. The theory Walker posited was that patients with PTSD have an increased amount of noradrenaline. This causes them to have difficulty achieving and maintaining REM sleep dreaming, thus hindering the separation of emotion from memories. So, Walker thought that something was needed to lower the noradrenaline levels, and lucky enough, another researcher found that the drug prazosin fulfilled this need. Prazosin had been prescribed to PTSD patients for their blood pressure, but a side effect was that their nightmares decreased. This was due to the prazosin suppressing noradrenaline levels in the brain (Walker 212). The mechanisms behind the cycle of nightmares in people with PTSD, as well as their inability to separate the emotion from their traumatic experiences, is still not completely known to researchers (Walker & van der Helm, 2009). However, one idea is that the amygdala’s function is altered, and this alteration is intensified during REM sleep. This in turn helps to increase the dysphoric nightmares people with PTSD experience (Scarpelli et al., 2019).

            MDD and its relating sleep abnormalities also have an effect on the overnight therapy theory. People with MDD tend to be affected by sleep disorders, such as insomnia. Insomnia can increase the risk for a depressive episode through its effect on REM sleep: REM sleep latency is reduced, the first REM sleep period is prolonged, and REM sleep density is increased. These can cause people with MDD to experience stimuli in a more negative, intense context, leading to the increased encoding and strengthening of negative emotional memories compared to neutral or positive emotional memories. This can create an imbalance in the memories people with MDD encode, making it more difficult to encode and strengthen positive emotional memories. While this is not a direct effect on the overnight therapy and one’s ability to strip emotions from memory, sleep disorders in people with MDD cause their emotional memories to change and favor the negative emotional memories one makes (Walker & van der Helm, 2009). While there has been research done on mental illnesses such as PTSD and MDD in relation to the overnight therapy theory, there are still many unknowns behind the mechanisms of them.

            REM- sleep dreaming has a much larger role than providing a virtual reality for us to explore in our sleep filled with bizarre images and experiences. Without REM-sleep dreaming, we would most likely remember emotional experiences in the same way we experienced them and with the same emotions. So next time you have a crazy dream, remember that it may be part of your overnight therapy, and could be separating the emotions from a memory!

References

Cartwright, R., Agargun, M. Y., Kirkby, J., and Friedman, J. K. (2006). Relation of dreams to waking concerns. Psychiatry Research, 141, 261-270. doi: 10.1016/j.psychres.2005.05.013 

Scarpelli, S., Bartolacci, C., D’Atri, A., Gorgoni, M., and De Gennaro, Luigi. (2019). The functional role of dreaming in emotional processes. Frontiers in Psychology, 10, 1-16. doi: 10.3389/fpsyg.2019.00459

Walker, M. (2017). Why We Sleep: Unlocking the power of sleep and dreams. Scribner.

Walker, M. P. and van der Helm, E. (2009). Overnight therapy? The role of sleep in emotional brain processing. Psychological Bulletin, 135(5), 731-748. doi: 10.1037/a0016570