S leep quality and daytime sleepiness are salient and clinically relevant dimensions of sleep-wake function. Poor sleep quality and insomnia symptoms have been associated with worse health, increased health care costs and utilization, absenteeism from work, and increased risk for psychiatric disorders, including depression. 1 Daytime sleepiness has been associated with increased risk of motor vehicle accidents, worse physical health, and increased mortality risk. 2 Although sleep and sleepiness can be measured by objective means such as polysomnography (PSG) and the multiple sleep latency test (MSLT), these methods are often impractical as clinical screening or research tools. Self-report questionnaires are most commonly used to assess sleep quality and daytime sleepiness. Many different instruments have been developed to measure sleep quality, insom-nia, and daytime sleepiness (for review, see 3 ), but 2 of the most widely-used are the Pittsburgh Sleep Quality Index (PSQI) 4 and the Epworth Sleepiness Scale (ESS). 5,6 A search of the ISI Web of Knowledge Citation Index in January 2008 identified over 900 publications citing the PSQI, and over 1500 citing the ESS. Despite their widespread use, however, relatively little attention has been paid to how the PSQI and ESS relate to each other, or to other clinical and sleep measures.The PSQI is a 19-item self-rated questionnaire for evaluating subjective sleep quality over the previous month. The 19 questions are combined into 7 clinically-derived component scores, each weighted equally from 0-3. The 7 component scores are added to obtain a global score ranging from 0-21, with higher scores indicating worse sleep quality. The clinical and psychometric properties of the PSQI have been formally evaluated by several research groups. 4,[7][8][9] The PSQI has a sensitivity of 89.6% and specificity of 86.5% for identifying cases with sleep disorder, using a cut-off score of 5. Validity is further supported by similar differences between groups using PSQI or polysomnographic sleep measures. The PSQI has been translated into 48

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