Overview

• The 2025 event asks residents to switch off outdoor lighting from 22:00 through the night, or close blinds where indoor lights must remain on.
• Cities are participating, with Duisburg darkening the town hall, theater and power plant features while urging residents and businesses to cut facade and advertising lights.
• Citizen observations from 2011–2022 paired with satellite models showed near-10% yearly increases in sky brightness that correspond to a rapid loss of visible stars.
• Researchers report disruptions to migratory birds, fireflies and tree cycles, alongside human health risks linked to circadian disturbance such as hypertension, diabetes, cancer and depression.
• Practical fixes—shielding, dimming and warm-white lamps, as used in Fulda—can cut skyglow and immediately reveal more stars.