Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is a distinct disease entity with an estimated prevalence of 0.3-0.7% and more common in women (3:1 ratio). It can be diagnosed according to the Institute of Medicine (IOM) 2015 consensus definition using 3 major criteria and one of 2 minor criteria.

Diagnosis requires that the patient have the following three symptoms:

1. A substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities that persists for more than 6 months and is accompanied by fatigue, which is often profound, is of new or definite onset (not lifelong), is not the result of ongoing excessive exertion, and is not substantially alleviated by rest,
2. Post-exertional malaise,* and
3. Unrefreshing sleep*

At least one of the two following manifestations is also required:

1. Cognitive impairment* or
2. Orthostatic intolerance

Note* Frequency and severity of symptoms should be assessed. The diagnosis of ME/CFS should be questioned if patients do not have these symptoms at least half of the time with moderate, substantial, or severe intensity.

Currently, individually tailored therapy with emphasis on cognitive behavioral therapy and graduated activity therapy is considered the therapy of first choice, although their effectiveness has been critically questioned in recent years. There are often frustrating treatment courses, a larger proportion of partial remissions, a significantly smaller proportion of full remissions and return to work.

The study aims to evaluate patients of the outpatient service for chronic fatigue at the Department of Consultation-Liaison Psychiatry and Psychosomatic Medicine, University Hospital Zurich, Switzerland, in the context of a group therapy for the treatment of CFS/ME in respect to the response to different, non-drug based therapeutic procedures and to gain knowledge about the effects of the therapy.

The study is a clinical comparative study of therapeutic procedures/interventions without the use of drugs or a medical product. The interventions are Acceptance Commitment Therapy (ACT) and Micro Breaks in Everyday Life (MBEL) adapted to CFS/ME. The collection of biological samples (saliva, blood) and health-related personal data (actigraphy, psychometric data from questionnaires) is associated with minimal risks and burdens.

Inclusion Criteria:

• Diagnosis for CFS/ME
• Psychiatric clinical stability in the past 3 months:
• No diagnostic change to other categories of the International Classification of Diseases (ICD-10)
• No psychiatric inpatient treatments
• No psychiatric emergency treatments
• No suicide attempts
• Possession of internet access
• Sufficient skills to use electronic devices
• The willingness to engage in the described therapeutic procedures or interventions (ACT, MBEL)

Exclusion Criteria:

• Insufficient knowledge of German
• Severe psychiatric disorders (e.g. personality and posttraumatic stress disorders, dissociative and psychotic disorders, intelligence reduction, untreated attention deficit hyperactivity disorder) and acute suicidal tendencies
• Untreated or severe internal medicine disorders e.g., thyroid dysfunction, central and obstructive sleep apnea syndrome (i.e., apnea-hypopnea index >15 and/or "high-risk group for obstructive sleep apnea" according to the Berlin Questionnaire)
• Cardiovascular disease such as chronic heart failure
• Severe or untreated neurological diseases (e.g. Parkinson's disease, dementia, restless legs syndrome, narcolepsy)
• Alcohol and drug dependence
• Initiation of psychopharmacotherapy at a dosage provided for guideline-appropriate treatment of a mental disorder according to the Drug Compendium in the past 3 months
• Start of other psychotherapy procedures in the last 3 months
• Other parallel therapy methods (e.g. acupuncture, qigong, osteopathy)
• Somatic (sleep-disrupting) treatments, cortisone treatment, or radio-/chemotherapy in the last 6 months