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

Abstract

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.

Figure 1

Histogram of changes in insomnia-duration subtype between first and last completed interview. One unit corresponds to a change into the next more- or less-persistent insomnia subtype. Categories are no insomnia, occasional brief insomnia, recurrent brief insomnia, 2- to 3-week insomnia, and 1-month insomnia. See text for further details.

Figure 2

Longitudinal relationships between insomnia and depression. Cross-sectional and longitudinal relationships between “pure” insomnia (including insomnia symptoms and occasional, recurrent brief, 2- to 3-week insomnia, and 1-month insomnia) “pure” depression (including major depressive episonde, minor depression, dysthymia, and depressive symptoms) and insomnia comorbid with depression (insomnia + depression) were analyzed using log-linear models. The strength of the associations is represented by the different line styles, as indicated in the figure. Numbers represent odds ratios. *Indicates that the odds ratio between depression in 1988 and 1993 was 0 and thus not interpretable. Panel a: longitudinal associations within pure insomnia, insomnia with comorbid depression, and pure depression conditions. Panel b: Longitudinal associations between pure insomnia and the other 2 conditions, as well as longitudinal associations between pure depression and the other 2 conditions. Panel c: Longitudinal associations between insomnia comorbid with depression and the other 2 conditions.