Insomnia and depression: clinical associations and possible mechanistic links.

Chronic insomnia is highly comorbid with multiple psychological and physical disorders, but most notably depression. While insomnia is now viewed as a risk factor for depression, and not just a 'symptom' of the disorder, the reasons for this change in classification (from symptom to risk factor) have yet to be clearly articulated. This is one goal of the present review. Furthermore, efforts to identify the mechanisms, by which insomnia is related to increases in depression is a burgeoning area of research. Here, our second goal is to highlight several potential mechanisms that can be targets for future research.