Introduction and purpose. University students often show unhealthy sleep practices, including irregular sleep habits, daytime napping, alcohol use, or using internet before going to sleep. Poor sleep hygiene is related to a variety of ill-health effects, including metabolism dysregulation, cardiovascular problems, memory and attention problems and poor academic performance. However, research on sleep practices mostly focuses on the consequences of poor sleep behavior, and seldom addresses its determinants. The purpose of this study was to test if the constructs of the Theory of Planned Behavior (TPB) predicted (un)healthy sleep behaviors of Belgian university students. Method. An online questionnaire was used to assess the frequency of irregular sleep times, daytime napping, alcohol use before sleep, and engaging in internet or video watching before bedtime, as well as the attitudes, perceived norms, perceived control and intentions towards these behaviors, amongst 1006 undergraduate students enrolled at a French-speaking university (70% females).Principal Component analysis (PCA) and internal consistency analysis (Cronbach alpha) was used to establish the validity and reliability of the scales representing the dimensions of the TCP for reach behavior. Results. Multiple regression analyses showed that perceived advantage and disadvantage, perceived norms and perceived control significantly predicted 21% of the intention to avoid irregular sleep patterns, 74% of the intention to take daytime naps, 27% of the intention to use internet or watch videos,and 45% of the intention to consume alcohol before bedtime. Intentions and perceived behavioral significantly predicted self-reported irregular sleeping times (R²=.15), daytime napping (R²=.36), internet use or video watching before bedtime (R²=.23), and alcohol use before bedtime (R²=.51). Multivariate analysis of variance revealed significant for the four sleep hygiene behaviors with regard to gender, study type and type of residence. Conclusion The TPB ...