Illumination is a very subtle decorating tool. It involves controlling a chaotic and unruly element. You are forcing something without form to conform to the regulations of human design. Yet there is no visual art without light, and so considerations must be made for this temperamental element when setting up an environment.
Light can make a room seem active or mellow. It can change the very nature of experiences within the space. Night clubs use light to create hypnotic effects, restaurants use lights to make you feel relaxed, and yes even hungry. Illumination is a powerful tool that you can use to gain a small amount of control over the world around you.
In a dark environment, the world feels laid back, relaxed, and comfortable. Humans are not nocturnal, and so our bodies react to lower lighting. We get sleepy, or at least mellow. This is a universal symptom of the human condition, one which is often used in interior decorating when making a choice between a calm or active atmosphere.
Dimmer lights and dimmer switches are one of the easiest ways to control lighting in a room. They allow you to slip the atmosphere from relaxed to exuberant with the flick of a wrist.
Another option is candles or small table lamps, which allow you to create tiny points of light through the space. The great thing about these options is that they have the added benefit of casting attractive shadows, which appear as moving, flickering pictures on the wall.
Complexity of lighting is one of the key factors in controlling this element. The more sources of light you have, the more control you will have over the entire environment. Having half a dozen dim lamps spread through a room gives you a lot more lighting options than just having a single bright piece in the space.