Superluminova is a special substance that glows in the dark after being exposed to light. It needs to be charged with light to glow. The brightness and duration of the glow depend on several factors, including the duration of exposure to light. Bright LED light, for example, works better than natural light on a cloudy day. The angle also matters; the watch charges less when it's moving on your wrist compared to when it's directly under a light source.