In healthy subjects, presence of the C allele of rs1801260 has been associated with a greater disruption of sleep patterns following stressful life events, and a few studies in mood disorder patients found that this variant was associated with sleep change only in case of prior stressful experiences (10) concluding that environmental stress may increase vulnerability to circadian rhythm disruption (10) and suggesting an interaction between CLOCK variants in shaping individual risk for deleterious effects of environmental stress including depression (47). This evidence concerns the gene CLOCK and major depressive disorder.