The main positive findings refer to associations between selected polymorphisms and: 1) chronotype with the ARNTL gene (rs11824092 and rs1481892) and the CLOCK (rs1268271) 2) sleep duration with the CLOCK gene (rs3805148) and the TIM gene (rs2291739) 3) daytime dysfunction with the PER3 gene (rs228727, rs228642, rs10864315) 4) subjective sleep quality with the ARNTL gene (rs11824092, rs1982350) 5) sleep disturbances with the ARNTL gene (rs11600996) We also found the significant epistatic interactions between polymorphism of the PER3 gene (rs2640909) & the CLOCK gene (rs11932595) and following sleep quality variables: sleep duration, habitual sleep efficiency and subjective sleep quality.