Introduction: Sleep is one of the most important factors affecting the quality of life. More than 50% of Polish adults have sleeping disorders, and young adults are the ones particularly vulnerable to this. This is why the analysis of the predictors of sleep quality, such as sleep hygiene knowledge and dietary knowledge, in young adults is a very important topic, especially after the COVID-19 pandemic, which may have affected sleeping habits. Material and Methods: 402 young adults (mean age 28.12 ± 6.08 years old) were enrolled in the study during the COVID-19 pandemic. Sleep quality was assessed with the Pittsburgh Sleep Quality Questionnaire, while dietary knowledge and sleep hygiene knowledge were assessed with dedicated questionnaires. The participants were also asked about the use of specific dietary supplements. Results: The analysis showed that sleep hygiene knowledge was significantly associated with sleep length (R = −0.17, p = 0.003) and overall sleep quality (R = −0.17, p = 0.005), while dietary knowledge significantly correlated with time to fall asleep (R = −0.12, p = 0.026). The regression analysis revealed that sleep hygiene knowledge was a significant negative predictor of sleep quality impairment (β = −0.13, p = 0.028). Significant results were also obtained for the use of supplements (β = −0.20; p = 0.001) and the use of medications (β = −0.32, p = 0.001), which were negative predictors of sleep quality impairment. Conclusions: This study showed that increased sleep hygiene knowledge promoted improved sleep length and overall sleep quality. In addition, dietary knowledge significantly correlated with time to fall asleep. On the other hand, decreased sleep quality was observed in participants who used medications or dietary supplements.