Each night, as the world quiets, the sleeping brain undertakes a kind of housekeeping — and new research suggests that when the deepest stages of that rest are lost, a dangerous residue accumulates. Scientists have found that disruptions to slow-wave sleep correlate with the buildup of tau protein, a molecular signature of Alzheimer's disease, pointing to sleep not as passive downtime but as an active defense against neurodegeneration. The findings invite a quiet reckoning: in a culture that routinely sacrifices rest, the long-term cost may be written into the very architecture of the aging mi