In the quiet hours when the body is meant to restore itself, light — long taken for granted as a passive presence — may be exacting a toll on the heart. New research links nighttime light exposure during sleep to measurable changes in cardiovascular function, suggesting that the darkness we have surrendered to glowing screens and ambient city light is not merely aesthetic but biological. The remedy, unlike so many modern health prescriptions, asks nothing more than the simple, ancient act of letting the night be dark.