Keep indoors, it may be quite challenging to provide bonsai plants with the right environment, particularly regarding light, which they use for photosynthesis.
If your tree starts to weaken and the leaves turn yellow and start to fall, then it may be a sign of poor light. If the leaves begin to wither, dim in colour, drop or dry, then the tree could be getting too much light.
The length of light exposure is a factor to consider—your bonsai will need both light and dark periods each day. On average, a tree needs around twelve hours of a certain amount of light and a minimum of ten to twelve hours of darkness to help the tree to rest.
South-facing windows can usually provide bright daylight (the type of light most plants appreciate). But how close or far your plant is to the window will drastically change the amount of light it will receive—this amount is usually calculated in lux (lumens per square meter).
If you keep your bonsai indoors, it will be at a room temperature, which gives to the tree trigger to start its growing phase. But to allow your tree to photosynthesize, it will need a minimum of 2,000 lux, or the above issues may start to rise—it has the right temperature to grow but not the right amount of light.
You can easily check the amount of lux your bonsai receives by purchasing a little light meter to help you estimate if the tree is getting enough light or needs extra support. To make up for deficiencies, several different kinds of lights partially or fully emulate the spectrum (rainbow band of colours) of light that the plants need to grow—fluorescents or LEDs, to name a few. The light choice is quite individual because it will be based on the type of plant collection and surface to cover.