Prevalence, course, and comorbidity of insomnia and depression in young adults

Study Objectives: (1) To describe the prevalence and prospective course of insomnia in a representative young-adult sample and (2) to describe the cross-sectional and longitudinal associations between insomnia and depression. Design: Longitudinal cohort study. Setting: Community of Zurich, Switzerland. Participants: Representative stratified population sample. Interventions: None. Measurements and Results: The Zurich Study prospectively assessed psychiatric, physical, and sleep symptoms in a community sample of young adults (n = 591) with 6 interviews spanning 20 years. We distinguished 4 duration-based subtypes of insomnia: 1-month insomnia associated with significant distress, 2- to 3-week insomnia, recurrent brief insomnia, and occasional brief insomnia. The annual prevalence of 1-month insomnia increased gradually over time, with a cumulative prevalence rate of 20% and a greater than 2-fold risk among women. In 40% of subjects, insomnia developed into more chronic forms over time. Insomnia either with or without comorbid depression was highly stable over time. Insomnia lasting 2 weeks or longer predicted major depressive episodes and major depressive disorder at subsequent interviews; 17% to 50% of subjects with insomnia lasting 2 weeks or longer developed a major depressive episode in a later interview. "Pure" insomnia and "pure" depression were not longitudinally related to each other, whereas insomnia comorbid with depression was longitudinally related to both. Conclusions: This longitudinal study confirms the persistent nature of insomnia and the increased risk of subsequent depression among individuals with insomnia. The data support a spectrum of insomnia (defined by duration and frequency) comorbid with, rather than secondary to, depression.