The secondary objectives are: observe the possible correlation between the degree of DNA methylation and the intensity of acute postoperative pain, observe the possible correlation between the degree of DNA methylation and the incidence of postoperative nausea and vomiting, observe the possible correlations between the degree of DNA methylation and the gene expression of μ opioid receptor, observe the possible correlations between the degree of DNA methylation and the expression of genes mainly involved in methylation processes, compare the degree of DNA methylation in the preoperative phase of the different patients involved in the study.