To examine whether objective sleep parameters are associated with cognitive function (CF) in patients with major depressive disorder (MDD) with chronic insomnia (CI) and whether the severity of these disorders is related to CF.

Thirty patients with MDD with CI attending a tertiary care institution underwent two consecutive nights of polysomnographic (PSG) recording and a battery of neuropsychological tests, which included episodic memory, sustained attention, working memory, and executive function. The severity of MDD and CI was assessed by clinical scales. We examined the relationship between PSG parameters and CF, as well as whether the severity of the disorders is related to CF.

Linear regression analysis revealed that total sleep time (TST) was positively associated with higher learning and recall of episodic memory, as well as better attention. Slow-wave sleep (SWS) showed a positive association with better working memory. Furthermore, wake after sleep onset (WASO) was negatively associated with episodic memory and lower attention. No significant relationships were found between the severity of MDD or CI with CF.

Both sleep duration and depth are positively associated with several aspects of CF in patients with MDD with CI. Conversely, a lack of sleep maintenance is negatively related to CF in these patients. These findings could help identify modifiable therapeutic targets to reduce CF impairment.