In its true sense, the term Full Sun refers to crops growing in an open field without shade except before dawn and after dusk. Think of a corn field. In gardening literature the term is used for plants that require a minimum of six hours of direct, unimpeded sunlight per day. These plants require solar energy in order to photosynthesize and to reach maturity in a relatively short season. At the peak of summer, when days are longer, it may be that such plants receive eight to ten hours of direct sunlight per day. Outdoors, a south-facing planting spot, without shade from trees or structures, is the easiest way to achieve full sun conditions.
Indoors, full sun conditions can be achieved using artificial lights. Both T5 fluorescent tubes and LED lights can provide bright, full spectrum light to simulate sunlight. Typically, the light source is kept within a few inches from the tops of plants or seedlings.