Schizophrenia and depression: The relation between sleep quality and working memory.

Sleep is known to be markedly disturbed in patients with depression, but in patients with schizophrenia these problems are underestimated. This research aimed to determine if a relationship existed between sleep problems in patients with schizophrenia and with depression and their reduced working memory (WM) performance. Thirty outpatients with schizophrenia, 30 outpatients with depression, and 30 healthy control participants were enrolled in this study. All participants completed a sleep questionnaire (i.e., Pittsburgh Sleep Quality Index (PSQI)), two simple WM tasks tapping only its storage component (i.e., digit span forward and backward task), and two complex WM tasks tapping both its storage and processing components (i.e., letter-number sequencing and reading span task). The results showed that neither psychiatric group differed from the healthy controls on simple WM tasks. Patients with schizophrenia did not differ from those with depression in the performance of simple or complex WM tasks. However, patients with schizophrenia, and, to a lesser degree, patients with depression performed significantly worse than the healthy control participants on complex WM tasks, which was visible in lower WM scores for patients with depression and in slower information processing, as well, for patients with schizophrenia. Finally, a significant negative relationship was found between the PSQI score and the reading span task scores; thus, participants with worse performance tended to report more sleep problems. To conclude, sleep needs to receive more priority when treating patients with depression and especially patients with schizophrenia because better sleep improves (working) memory performance and daily functioning.