Co-occurrence of severe fatigue and insomnia: implications for the outcome of cognitive behavioural therapies

Background: Cognitive behavioural therapy for fatigue (CBT-F) and insomnia (CBT-I) are effective therapies. Little is known on their effectiveness when severe fatigue and insomnia co-occur. Aims: This observational study investigated whether the co-occurrence of fatigue and insomnia influences the outcomes of CBT-F and CBT-I. Furthermore, it was determined if changes in fatigue and insomnia symptoms are associated, and how often the co-occurring symptom persists after CBT. Method: Patients with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS, n = 241) received CBT-F and patients with insomnia disorder (n = 162) received CBT-I. Outcomes were fatigue severity assessed with the subscale of the Checklist Individual Strength (CIS-fat) and insomnia severity assessed with the Insomnia Severity Index (ISI). In each cohort, treatment outcomes of the subgroups with and without co-occurring symptoms were compared using ANCOVA. The association between changes in insomnia and fatigue severity were determined using Pearson’s correlation coefficient. Results: There were no differences in treatment outcomes between patients with and without co-occurring fatigue and insomnia (CBT-F: mean difference (95% CI) in CIS-fat-score 0.80 (−2.50-4.11), p = 0.63, d = 0.06; CBT-I: mean difference (95% CI) in ISI-score 0.26 (−1.83-2.34), p = 0.80, d = 0.05). Changes in severity of both symptoms were associated (CBT-F: r = 0.30, p < 0.001, CBT-I: r = 0.50, p < 0.001). Among patients no longer severely fatigued after CBT-F, 31% still reported insomnia; of those without clinical insomnia after CBT-I, 24% remained severely fatigued. Conclusion: CBT-F and CBT-I maintain their effectiveness when severe fatigue and insomnia co-occur. Changes in severity of both symptoms after CBT are associated, but the co-occurring symptom can persist after successfully treating the target symptom.