Psychological inflexibility as a predictor associated with insomnia.

Psychological flexibility has recently attracted the attention of researchers in the field of sleep disorders; therefore, in the study, psychological flexibility was evaluated as a predictor or factor related to the presence/severity of insomnia. We included 2218 individuals selected from the randomized-control trial for behavioural therapy for insomnia and cross-sectional studies, including 1797 individuals with insomnia and 421 controls without insomnia. All participants completed the DSM-5-based insomnia diagnosis interview, Insomnia Severity Index, Hospital Anxiety and Depression Scale, and Acceptance Action Questionnaire-II. Linear regression and multinomial logistic regression models were used. Sex, education, occupation, marital status, anxiety, depression and psychological inflexibility were possible predictors or factors associated with the severity of insomnia. Multivariate linear regression analysis demonstrated that sex (𝛽 = 0.88; t = 2.80; p = 0.005), depression (𝛽  = 0.41; t = 10.7; p < 0.001), anxiety (𝛽  = 0.58; t = 14.1; p < 0.001) and psychological inflexibility (𝛽  = 0.09; t = 5.07; p < 0.001) were predictors of insomnia. The results of the multinomial logistic regression demonstrate that, in comparison to the absence of insomnia, insomnia at all levels (mild, moderate and severe) was associated with sex, anxiety and depression. Psychological inflexibility was only associated with severe insomnia (odds ratio = 1.04). These findings are important from a public health perspective because behavioural strategies designed to treat insomnia with a focus on psychological flexibility are low-cost and may help improve sleep quality in adults with insomnia, which also influences mental health.