The most studied form of epigenetics is DNA methylation and several studies have investigated the link between the methylome and body weight. In paper I we analyzed the methylation profile of whole blood in 46 subjects measured with Illumina 27K chip. We provide evidence that obesity influences age driven epigenetic changes. These identified markers may prove to be valuable biomarkers for the understanding of the molecular basis of aging, obesity and associated diseases. In paper II we studied the effect of bariatric surgery, and subsequent weight loss, on methylation and relating this to normal weight controls. In paper II we found 115 promoters had altered methylation after surgery. Among these promoters, an enrichment for genes involved in metabolic processes was found (n=36, p<0.05). In addition, these 51 promoters was more similar after surgery to that of normal-weight controls, than it had been at baseline (p<0.0001). One of the major comorbidities of severe obesity is obstructive sleep apnea and lack of sleep is highly correlated with obesity. Paper III shows how acute sleep deprivation increases portion size and affects food choice in 16 young men. In paper VI, whole genome DNA methylation profiles of whole blood was assessed following both conditions by the Illumina 450K methylation in the same trial as in paper III. This paper shows how sleep deprivation affects DNA methylation profiles of whole blood in a manner both dependent and independent on monocyte subpopulations. Hypothesis free genome wide analysis revealed differential methylation in ING5, a gene previously known to be differentially expressed in sleep deprivation.