The Pittsburgh Sleep Quality Index (PSQI) is the most common measure of sleep quality. Its questions refer to "usual" sleep habits during the last month. Considering how different sleep-wake behavior can be between workdays and work-free days, we hypothesized that sleep quality should show similar differences. We investigated these potential differences in a cross-sectional online study using the original and two adapted versions of the PSQI that replaced "usual" by explicitly referring to sleep on workdays or work-free days. Additionally, we investigated how these scores relate to chronotype and social jetlag assessed by the Munich ChronoType Questionnaire. Participants were recruited online, they had to be older than 18 years, following regular weekly work schedules, and they should not be shift workers. Repeated-measures analysis of variance was used to compare the three different versions of the PSQI (usual, work, and work-free). To find out whether PSQI score differences could be predicted by chronotype and/or social jetlag a mediation analysis was carried out. Workday PSQI scores were similar to the original "usual" scores, two points higher than the PSQI score on work-free days and above the cutoff designating poor sleep quality. PSQI components and time variables also differed between workdays and work-free days. Chronotype correlated with the difference between PSQI scores on workdays and on work-free days, an association mediated by social jetlag. Our results suggest that the original PSQI predominantly reports sleep quality on workdays and that work schedules may affect sleep quality. The mediation of social jetlag on the association of chronotype and PSQI score differences could mean that not chronotype per se, but rather the collision of an individuals' chronotype with fixed work schedules explains the differences between sleep on workdays and work-free days. Understanding how sleep quality differs between workdays and work-free days, how this difference can adequately be assessed through directing participants to focus on their sleep specifically on workdays vs. work-free days, and how circadian factors modulate this difference, is crucial to improve sleep quality.
