Study objectives Insomnia, depression, and anxiety show high rates of comorbidity and functional impairment. Transdiagnostic symptom interactions may be implicated in this comorbidity. This network analysis sought to assess how symptoms of insomnia, depression, and anxiety may interact and individually predict impairment across several domains for individuals with insomnia.
Methods Baseline psychometric data from a randomised controlled trial were analysed (N = 1711). A regularized partial correlation network was estimated from the symptom data. Centrality (symptom connectivity), community structure (symptom clustering), and bridging (inter-community connectivity) were assessed. The replicability of the network model was assessed via confirmatory analyses in a holdout sample. Separately, Shapley values were estimated to determine the relative importance of each symptom in predicting functioning (i.e., psychological wellbeing, psychosocial functioning, and physical health impairment).
Results The most connected nodes were uncontrollable worrying; trouble relaxing; and depressed mood/hopelessness. Five communities were identified with trouble relaxing identified as the bridge symptom between communities. The model showed good fit in the holdout sample. Low energy and depressive affect symptoms (feelings of failure/guilt; depressed mood/hopelessness; anhedonia) were key predictors in the relative importance analysis across multiple domains of impairment.
Conclusion Trouble relaxing may be of clinical and transdiagnostic significance in the context of insomnia. In terms of how symptoms relate to functioning, it was clear that, while low energy and feelings of failure/guilt were prominent predictors, a range of symptoms are associated with functional impairment. Consideration of both symptoms and functional impairment across domains may be useful in determining targets for treatment.
Sleep disturbances are a prime target for transdiagnostic analysis. The far-reaching relevance of sleep in psychopathology is wellestablisheddin fact, it has been proposed that sleep quality should be treated as a fundamental dimension in mental health under the US National Institute of Mental Health's (NIMH) Research Domain Criteria (RDoC) [1,2]. Sleep disturbances are observed across many (possibly most) mental disorders [3,4]. Sleep disturbances, including insomnia, feature in DSM-5 diagnostic criteria for both anxiety and depressive disorders, and an estimated 40% of insomnia patients have a comorbid mental health condition . Insomnia is associated with particularly high rates of comorbidity with depression and anxiety  and significantly predicts onset of these two disorders (anxiety disorders OR 3.23; depression OR 2.83). This association may be causal where sleep difficulty has been shown to predict the development of mental health conditions, including anxiety and depression . As such, sleep is arguably one of the most critical psychophysiological processes in neural and mental health [1,4].
1.1. The network analytic literature
While disorders assume a common cause, the network approach to psychopathology proposes that symptoms are constitutive (not merely passively reflective) of disorders; disordered states may emerge from causal interaction/reinforcement between symptoms (and relevant external factors) . Symptoms are thus treated as autonomous causal entities: worry may cause trouble sleeping, in turn causing fatigue [8,9]. Then, symptoms are noninterchangeable in that, naturally, different symptom profiles will show different dynamics, and some symptoms will prove more central or interconnected than others, potentially maintaining an episode . Symptoms may cluster into communities , and symptoms may act as bridges between these communities identified in a manner statistically agnostic to theoretical constructs (e.g., initial diagnostic categorisation) . Network analysis is thus particularly suited for assessment of transdiagnostic symptom interplay (i.e., in comorbidity). It may provide insight into how comorbidity is maintained through central nodes, pathways, and bridging structures.
In the context of insomnia, symptoms of insomnia, depression, and anxiety arranged themselves along distinctly transdiagnostic dimensions (such as those relating to psychomotor disturbance). Individual symptoms were highly variable in terms of their centrality, inter-community influence (bridging), and relative importance vis-a-vis functional impairment and wellbeing/life satisfaction levels. Overall, findings further substantiate the value of the burgeoning symptomic approach in psychopathology. Sole reliance on sum scores and the assumption of symptom interchangeability in diagnosing and treating these disorders may be problematic. Assessment of transdiagnostic symptom interplay alongside consideration of how symptoms variably relate to impairment and QOL could help inform targets for clinical intervention. This secondary data analysis also bolsters calls for the investigation of sleep and psychomotor symptoms under the RDoC, i.e., as basic dimensions of psychopathology.1,2,107