A study from Tulane University draws a quiet but consequential line between the light we leave on at night and the health of our hearts. What the body needs in darkness — melatonin, circadian coherence, restorative rest — is steadily undermined by even modest bedroom light above 3 lux, setting in motion a cascade that touches cardiovascular function, metabolism, and immunity. The finding is less a discovery than a clarification: the body's ancient relationship with light and dark was never meant to be overridden, and the cost of doing so accumulates slowly but measurably across a lifetime.