Insomnia is a predictor of depression: A meta-analytic evaluation of longitudinal epidemiological studies.

Objectives: In many patients with major depression, symptoms of insomnia herald the onset of the disorder and may persist into remission or recovery, even if the depression is adequately treated. Several studies have raised the question whether insomniac symptoms may not only be early signs of the disorder, but constitute an independent risk factor to become depressed. Up to now, however, no systematic evaluation of longitudinal epidemiological studies is available evaluating the role of insomnia as a risk factor for depression. The present analysis was conducted to quantitatively evaluate if insomnia may indeed predict future depression, and if such a relationship is evident independent of age. Method: Reference databases were searched for longitudinal epidemiological studies simultaneously investigating insomniac complaints and depressed symptoms/psychopathology with at least a one year interval between first and second measurement. The odds ratios (for insomnia at baseline to predict depression at follow-up) were tabulated with 95% confidence intervals (CI), as indices of the predictive value of insomnia for future depression. The logarithms of the odds ratios were combined using a fixed-effects model when heterogeneity within studies was not found. When heterogeneity was found, a random effects model was used and subgroups of studies were considered in order to identify sources of variation and contexts in which the results could be generalised. Results: Twenty studies met inclusion criteria. Considering all studies together, heterogeneity was found. The random-effects model showed an overall odd ratio of 2.61 (CI: 1.97–3.45). Age explained the largest amount of variability in the studies. Longitudinal studies conducted on adults (18–60 years) and on elderly (> 60 years) were not heterogeneous when regarded separately. The fixed-effects model showed respectively an odd ratio of 2.095 (CI: 1.72–2.55) for the adults, and of 1.920 (CI: 1.56–2.37) for the elderly. Conclusions: Adult individuals with insomniac symptoms have a twofold risk to develop depression, as compared to people with no initial sleep difficulties. It is postulated that dissemination of early and easy accessible treatment programs for insomnia might reduce the risk for developing depression in the general population and thus be considered a helpful general preventive strategy for mental health.