CLOCK and cancer: Several plausible hypotheses have been proposed to explain the link between circadian clock disruption and cancer, among them: the suppression of nocturnal peak of melatonin after exposure to light at night; immune system alterations as a consequence of sleep deprivation; shift in the ratio between anti-tumor and pro-tumor cytokines, induction of inflammation response, modifications in the levels of appetite-regulating hormones, internal desynchronization and disturbances in the regulation of several clock genes controlling the cell cycle, apoptosis, DNA damage repair and cell proliferation.