Sleep disturbances are considered an underlying neurobiological mechanism of depression, a condition deemed a leading nonfatal cause of disability by the World Health Organisation. Approximately 50 to 70 million adults in the United States suffer from a sleep disorder, and 40% of all patients with insomnia have a coexisting psychiatric condition.
To learn more about the impact of sleep disturbances on patients with depression, Psychiatry Advisor spoke with Elaine Boland, PhD, research psychologist at the Corporal Michael J. Crescenz VA Medical Center, Philadelphia, Pennsylvania; David Smith, PhD, assistant professor of psychology, Temple University, Philadelphia, Pennsylvania; and Michael Grandner, PhD, director of the Sleep and Health Research Program and assistant professor, department of psychiatry, University of Arizona College of Medicine – Tucson.
Reward Processing and Depression
It is medically acknowledged that depression and sleep disturbance go hand-in-hand, but what might the underlying mechanism behind this relationship be? Research suggests that a deficit in reward processing affected by sleep disturbance may be the answer. Reward processing encompasses the biological and behavioral functions that facilitate the acquisition of rewarding stimuli. By responding to rewarding stimuli, individuals learn from rewards, anticipate future rewards, and engage in goal-directed behavior.
According to Elaine Boland, PhD, “Some studies have shown that reward processing deficits are often apparent in the context of anhedonia, which is a cardinal symptom of depression that translates to a reduced ability to experience pleasure and/or reduced motivation to engage in pleasurable or valued activities. More work needs to be done to understand if and how reward processing may play a causal role in depression.”
David Smith, PhD, sheds further light on the physiological relationship between depression and reward processing, saying, “When healthy individuals experience or anticipate a reward, we tend to see increased activation in the ventral striatum, ventromedial prefrontal cortex, and orbitofrontal cortex, but individuals with depression appear to have aberrant neural responses to reward. For example, our recent meta-analysis, led by Tommy Ng, showed that individuals with depression have reduced (or blunted) striatal responses to reward and elevated orbitofrontal cortex responses to reward.”
An Integrated Sleep and Reward Processing Model
Dr Boland’s research4 on sleep, reward processing, and depression provides evidence that sleep disturbances are pervasive in depression, treatment of sleep disturbance is associated with improvement in depression symptoms, reward processing is impaired in depression, and insomnia-like sleep disturbance is associated with disturbance in reward processing.
The hypothesis is that these findings reflect the workings of an integrated process, whereby sleep disturbance can influence reward-related effort, motivation, and learning functions.
“The relationship between depression and reward processing is a complex one, as there are many different types of reward processes in the human brain, and depression is not unilaterally associated with abnormalities in all areas of the reward function,” says Dr Boland. Further research is clearly needed, but the relationship has significant implications for the treatment of depression. In particular, treatments that do not also target sleep disturbance may be less successful. For example, cognitive-behavioral therapy for insomnia has been found to improve symptoms of both insomnia and depression in a small sample.
Sleep Subtypes and Depression
The relationship between sleep and depression is complex because sleep is multifaceted and can involve numerous types of disturbance. This point raises the question of which types of sleep disturbance are more likely to be related to depression or deficits in reward processing.
Michael Grandner, PhD, and colleagues have identified 3 sleep disturbance symptomatic subtypes and the depression symptoms they appear to affect most:6
Sleep disturbance subtype
Sleep symptoms: the physical difficulties related to sleep disturbance such as falling asleep and staying asleep.
More associated with physiologic depression symptoms, such as appetite changes and psychomotor dysregulation.
Daytime symptoms: the functional difficulties related to sleep disturbance, such as having a lack of energy and fatigue during the day.
More associated with cognitive depressive symptoms and experiential symptoms in general, such as difficulty concentrating and depressed mood.
Perception symptoms: the psychological difficulties related to sleep disturbance, such as dissatisfaction, anxiety, and worry about sleep.
More associated with attributional depressive symptoms, such as feelings of worthlessness and self-dissatisfaction.
According to Dr Grandner, “Clinically significant sleep disturbances like insomnia disorder are associated with longer time to remission from depression as well as a greater likelihood of relapse.” Nonetheless, treating insomnia is rarely part of the treatment plan for depression. When it is, Dr Grandner points out that clinicians are treating sleep disturbance as they would a symptom, not a comorbid condition. This approach is detrimental to the patient, as the symptom of sleep disturbance and the disorder of insomnia are different.
Insomnia is a diagnosable medical condition with delineated criteria, which rarely resolves when comorbid issues such as depression are treated. In contrast, sleep disturbance as a symptom encompasses a variety of experiences and might be addressed as part of treatment, especially if depression is present.
“There is evidence to suggest that reward processing changes following depression treatment. For example, one study showed that using positron emission tomography to examine the effects of a dopamine antagonist (amisulpride) in patients with major depression increased reward-related striatal activation and corticostriatal connectivity,” Dr Smith noted, “In another line of work, a review synthesizing findings from studies examining the effects of noninvasive brain stimulation on major depression, such as transcranial magnetic stimulation and transcranial current stimulation, suggests that these techniques can be used to normalize responses to reward and improve clinical symptoms.”9
According to Dr Boland, there is consistent evidence that improvements in sleep can be linked to improvements in depressive symptoms, and longer time to remission in depression, or even relapse, may be related to clinically significant sleep disturbances. She noted that uncovering the relationship between sleep disturbances and the course of depression could play a role in developing “targeted psychotherapies and medications.” Furthermore, Dr Boland stated, “Since reward processing dysfunction is so frequently observed in depression and may play a key mechanistic role, understanding how sleep disturbance may relate to that core process could help us get closer to both improving and personalizing depression treatment.”
The benefits of exploring reward processing go beyond helping discover new treatments for sleep disturbance and depression. Deficits in reward processing are evident in several psychological disorders, including anxiety, substance abuse, personality disorders, attention-deficit/hyperactivity disorder, and psychotic disorders. Therefore, identifying at-risk individuals and tailoring interventions to target the reward processing system could represent an effective strategy to prevent later psychopathology.
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