Study objectives: Chronic insomnia tends to "wax and wane" over lifetime. The presence of residual insomnia symptoms is common, especially among naturally remitted individuals. This study aims to examine the features of these residual symptoms and their potential association with future relapse.
Methods: A population-based data set on the natural history of insomnia was used for this secondary analysis. Residual insomnia symptoms were investigated in those who had insomnia symptoms/syndrome at baseline and achieved full remission (according to predetermined diagnostic algorithm) within the following 1 year. Cox regressions were used to determine the hazard ratio (HR) of each residual symptom for predicting relapse in the next 4 years. The nature and severity of residual symptoms were examined with an extended version of the Insomnia Severity Index (ISI), which incorporates additional items on sleep quality and specific sleep-related daytime impairments (on daytime fatigue, cognitive functioning, mood, interpersonal relationship, and daily activities). In addition, the presence of depressive symptoms and medical conditions were controlled for in investigating risks of insomnia relapse.
Results: A total of 434 participants were included in this study (age ranges from 18 to 94; 65.9% female); 248 of them had relapsed within 4 years. The response rate ranged from 78% to 83%. The most frequently reported residual symptoms with at least moderate severity (ISI items ≥2 on 0-4 ISI item scale) were poor "Quality of sleep" (39.2 %), followed by "difficulty maintaining sleep" (DMS; 27%). The most common residual daytime impairments related to insomnia were fatigue (24.7 %), mood disturbances (23%) and cognitive disturbances (22.6%). After controlling for baseline insomnia and depression severity and concurrent physical diseases, impairments of cognition (HR = 1.46), poor quality of sleep (HR = 1.43), disturbed mood (HR = 1.39), being female (HR = 1.36), DMS (HR = 1.35), and fatigue (HR = 1.24) were significantly associated with insomnia relapse in the next 4 years. Moreover, residual poor sleep quality and daytime insomnia symptoms were independent of DMS in predicting relapse. Subgroup regressions according to sex showed that for male participants, residual cognition impairments (HR = 1.98) was the most significant predictors of future relapse, whereas residual DMS (HR = 1.46) significantly predicted relapse for women only.
Conclusion: A wide range of residual symptoms exists in individuals with naturally remitted insomnia. Notably, residual DMS is the most common residual nighttime symptom and the only nighttime symptom associated with insomnia relapse. Additionally, perceived poor sleep quality and cognitive, mood, and somatic impairments attributed to sleep disturbances are also related to future relapse. Attention to these residual symptoms when initiating insomnia treatment is warranted to minimize future relapse.
Keywords: insomnia; natural history of insomnia; natural remission; relapse; residual symptoms.
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