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