Introduction Pittsburg Sleep Quality Index (PSQI) is a widely used standardized instrument to assess sleep quality in clinical and research settings. Objective of the study was to translate the PSQI into Sinhala language and validate using a combined qualitative and quantitative approach.

Methods Every fifth patient aged 18-60 years who attended the out-patients department of a tertiary care hospital was recruited. PSQI was translated into Sinhala using a combined qualitative and quantitative approach. Internal consistency was measured using Cronbach’s alpha. Construct validity was assessed by comparing the scores in patients who were identified as having depressive disorder according to the Centre for Epidemiologic Studies Depression Scale (CES-D) and those without depressive disorder.

Results Forty-six participants with depression were compared with 159 non depressed controls. Mean PSQI component scores were significantly higher in depressed patients in 5 components. Factor analysis identified a single component explaining 53.53% of the variance. Cronbach’s alpha of 0.85 indicated a high internal consistency.

Conclusions The Sinhala translation of the PSQI is a valid and reliable tool to assess sleep quality.