As with any plants, the plants inside the mini-biospheres need light to survive. However, the terrarium should not be placed in direct sunlight (in this case the temperature inside can get too high). A place 1-2 meters away from the window would be ideal. Alternatively, you can place it in a dark spot, but under a lamp or spotlight (some biospheres come with their own LED light integrated). In this case, they are very decorative and light up a dark part of a room lifting the mood up and turning it cozy.