The Earth, seen from the night sky, reveals a magical vision. Cities and villas, offices and parking lots, industrial zones and monuments are all illuminated. But night is also a conductor, directing the entire living world. Science reveals that the starry sky is a powerful landmark for insects, mammals, plants and even humans. It is perhaps in the night that life first appeared, and it is thanks to the night that memories are consolidated during sleep. But for the first time on a planetary scale, the immemorial day/night cycle is being disrupted by artificial lighting, threatening the global ecosystem. Researchers around the world are sounding the alarm.