Comparing subjective and objective nighttime- and daytime variables between patients with insomnia disorder and controls – a systematic umbrella review of meta-analyses

Insomnia disorder is a prevalent health problem with adverse consequences for mental health and quality of life. Although insomnia disorder is defined in diagnostic systems as a subjective complaint about sleep, this understanding is not consistently applied in clinical settings. There remains a lack of clarity regarding the mechanisms underlying subjective sleep complaints and associated daytime impairments. This systematic umbrella review of meta-analyses aimed to compare subjective and objective health-related variables between patients with insomnia disorder and controls, characterizing insomnia comprehensively. After a systematic literature search, we included six meta-analyses comparing patients with insomnia disorder and controls in terms of subjective sleep (sleep diaries), objective sleep (polysomnography), peripheral brain-derived neurotrophic factor, cortisol, objective cognitive performance, blood pressure and EEG spectral power. The pattern of results suggests that differences between patients with insomnia disorder and healthy controls are largest and most consistent for subjective sleep. Objective between-group effect sizes were largest for unspecific stress markers such as brain-derived neurotrophic factor and cortisol. Variables expected to be abnormal in those with objective sleep deprivation, such as objective attention and alertness, showed minimal group differences. Also in terms of sleep, effect sizes for subjective variables were consistently larger than those for objective variables. Critical appraisal of the quality of the included meta-analyses using the AMSTAR 2 tool suggested moderate overall confidence in the results, whereby shortcomings in several critical domains such as pre-registration of the study protocol and justification for the inclusion of individual studies have been identified. Our findings highlight that insomnia disorder is characterized by large reductions in subjective sleep quality, in the absence of large objective alterations. This body of evidence supports a biopsychosocial conceptualization of chronic insomnia disorder with a strong psychological component.