Transitions® lenses darken when exposed to UV light. As UV from sunlight is typically not present indoors, the lenses will be clear.
The exception is Transitions® XTRActive® lenses, which react to both UV and visible light. This allows them to get extra dark outside and also when you're sitting in bright sunlight behind a window. Transitions XTRActive lenses are also designed to have an unnoticeable slight tint indoors to protect from harsh indoor light.