Levels of mood instability (MI) appear to be high in people with depression, but temporal precedence and possible mechanisms are unknown.

We tested hypotheses that: i] MI will be associated with a diagnosis of depression cross-sectionally; ii] MI will predict new onset and maintenance of depression prospectively; iii] the association between MI and depression will be mediated by sleep problems at baseline, new onset alcohol abuse and life events 6 months preceding new onset depression.

We used data from the National Psychiatric Morbidity Survey 2000 at baseline (N=8580) and 18 month follow-up (N=2413). Regression modelling controlling for socio-demographic factors, anxiety and hypomanic mood was conducted. Multiple mediational analyses were used to test our conceptual path model.

MI was strongly associated with a diagnosis of depression cross-sectionally (OR: 5.28 (95% CI, 3.67-7.59) p< 0.001). MI predicted depression inception (2.43 (1.03 – 5.76) p=0.042) after controlling for important confounders. MI did not predict maintenance of depression. Quality of sleep and severe problems with close friends and family significantly mediated the link between MI and new onset depression (23.05% and 6.19% of the link respectively). Alcohol abuse and divorce were not important mediators.

Mood instability is a precursor of a depressive episode but does not worsen the course. Interventions targeting mood instability and sleep problems have the potential to reduce the risk of depressio