Evaluating reduced cognitive flexibility as a link between Insomnia and future depression: A systematic review

Insomnia is a causal risk factor for depression, but the underlying mechanisms are unclear. Reduced cognitive flexibility, a component of executive function, is linked to both insomnia and depression and may mediate their relationship. This systematic review evaluated this potential mediation model by examining studies reporting associations among insomnia, cognitive flexibility, and depression.

A comprehensive search identified 35 studies reporting associations between insomnia and cognitive flexibility (a path) and 11 studies examining cognitive flexibility as a predictor of future depression (b path). No studies formally tested cognitive flexibility as a mediator of the insomnia-depression relationship (ab path). Studies were assessed for quality and capacity for directional and causal inference. Findings were synthesized to evaluate the mediation model.

For the a path, 35 studies reported 117 eligible associations. Longitudinal studies consistently reported null associations between insomnia and cognitive flexibility. Cross-sectional findings were also predominantly null. Significant associations generally did not withstand covariate adjustment or were observed only in older participants or those taking benzodiazepines. For the b path, 11 prospective cohort studies reported 37 eligible associations. Most associations were null, but significant associations emerged in participants without baseline depression, suggesting cognitive flexibility may relate more to depression risk than prognosis.

Current evidence does not support reduced cognitive flexibility as a mediator in the insomnia-depression pathway. Subjective cognitive concerns may offer a more promising explanatory mechanism. Future research should integrate objective and subjective cognitive assessments, test formal mediation models, and evaluate alternative pathways.