Research has linked disrupted sleep patterns to an increased risk of certain cancers, particularly breast and colorectal cancer, though the evidence remains mixed. Shift work offers some of the strongest evidence for this link, with long-term night shift workers showing higher cancer risks compared to those working regular hours. Sleep may influence cancer development through several pathways, including melatonin secretion, immune surveillance, and DNA repair. Recent findings show that circulating tumor cells—the seeds of metastasis—are predominantly released during sleep, adding a surprising layer of complexity to sleep’s role in cancer progression.