Sex Moderates the Mediating Effect of Physical Activity in the Relationship Between Dietary Habits and Sleep Quality in University Students

<b>Background/Objectives</b>: Diet and physical activity are key lifestyle behaviours associated with sleep quality, yet their combined and sex-specific associations remain insufficiently understood. This study examined the associations between dietary behaviours and sleep quality among university students and assessed whether physical activity formed part of an indirect statistical association between these variables, with sex considered as a moderator. <b>Methods</b>: A cross-sectional study was conducted among 418 students (199 males, 219 females) from the Wroclaw University of Health and Sport Sciences. Body height and body mass were measured using standard anthropometric procedures. Sleep quality (SQ) was registered with the Pittsburgh Sleep Quality Index (PSQI), dietary habits were assessed with the Questionnaire of Eating Behaviours (QEB) and physical activity (PA) was assessed with the International Physical Activity Questionnaire (IPAQ). Data-driven feature-selection methods were applied to identify dietary behaviours associated with sleep quality, which were combined into a Synthetic Dietary Behaviour Index (SDBI). A moderated mediation model, adjusted for body mass index (BMI), was then used to examine the statistical associations between dietary behaviours, physical activity, sleep quality, and sex. Sleep quality was modelled as a continuous PSQI score in mediation analyses, while the dichotomised PSQI category was used only for feature selection. <b>Results</b>: Machine-learning feature selection identified nine dietary behaviours statistically associated with sleep quality. Unfavourable behaviours-fast food, fried meals, sweetened beverages, energy drinks and alcohol-were linked to poorer sleep, whereas vegetables, curd cheese and wholegrain bread were associated with better sleep. Poor sleep was more prevalent among females (45.2% vs. 14.6%, χ² (1) = 65.4, <i>p</i> < 0.001). The mediation model indicated that physical activity formed part of a statistically significant but modest indirect association between dietary behaviour and sleep quality, with sex moderating the IPAQ → PSQI path (β = -0.45, <i>p</i> = 0.006). Indirect associations were significant for both sexes but stronger among females (males: β = 0.032, <i>p</i> = 0.021; females: β = 0.102, <i>p</i> = 0.004). <b>Conclusions</b>: Unfavourable dietary patterns and lower physical activity were statistically associated with poorer sleep quality, with a stronger indirect statistical effect observed among females. These findings support the relevance of integrated, sex-sensitive lifestyle approaches addressing both dietary behaviours and physical activity, while acknowledging the cross-sectional nature of the data.