Light is important to plants because it allows them to make their own food. This process, called photosynthesis, is when a plant absorbs light energy and combines it with water and carbon dioxide to create food -- starches and sugars -- as well as water and oxygen. Chlorophyll, the substance that makes plants green, is essential to this process in most cases. Without light, photosynthesis cannot occur.
A plant needs enough light to allow it to make adequate food to maintain itself and to allow it to grow. If the light is not sufficient, the plant's leaves will be smaller than they should and the color of the leaves will lack intensity. The plant will also get tall and leggy in appearance, with a long stem and few leaves. If the lack of light is ongoing, the plant will lose vigor over time, eventually deteriorating to the point that it dies.
According to Texas A&M University Extension, plants require a period of dark in order to grow properly. In most cases, the plants need at least eight hours of dark out of every day. Some flowering plants have even more specific requirements, as such plants as poinsettia and Christmas cactus will not bloom if they are exposed to light for more than 11 hours per day. The leaves on plants that get too much light begin to get pale. They may end up appearing burnt, and the leaves will ultimately turn brown and die. If the overexposure continues, the entire plant will succumb.
Texas A&M University does point out that some tropical plants can do well with constant light. These plants are those from at or near the equator that are used to extremely intense light 12 hours each day. When placed in an environment where there is constant lighting, such as in an office, the perpetual lights lack the intensity of the sunlight at the equator. The longer exposure time compensates for the diminished strength of the light, and these tropical plants will thrive under such conditions.