Scientists have discovered that breast cancer can quietly throw the brain’s internal clock off balance—almost immediately after cancer begins. In mice, tumors flattened the natural daily rhythm of stress hormones, disrupting the brain-body feedback loop that regulates stress, sleep, and immunity. Remarkably, when researchers restored the correct day-night rhythm in specific brain neurons, stress hormone cycles snapped back into place, immune cells flooded the tumors, and the cancers shrank—without using any anti-cancer drugs.