Is Insomnia the Cause or Result of Depression?

Matthew Walker exposes a decades-long clinical blind spot: insomnia was labelled "secondary" and left untreated, assumed to vanish once depression lifted. Prospective studies reversed the arrow. The 1989 landmark trial and two independent meta-analyses showed that persistent sleep complaints doubled the risk of later depression in people whose mood was fine at baseline. Trials pooling over sixty interventions revealed a dose-response chain: the more sleep improved, the more mood followed. Yet the effect is moderate, and heritability data suggest the arrow runs both ways.

You'll learn:

- Why the "secondary insomnia" label delayed treatment for years

- How dose-response patterns distinguish causation from coincidence

- What studies in communities without electricity reveal about the eight-hour myth