Being seen on your bike at night is essential for survival. If people in cars can't see you they can't avoid hitting you. You should always use lights at night. Its the smart thing to do. Here is another thing you can do to make yourself seen at night. You can get reflective tape at hardware, auto and cycling stores. It usually comes in strips or sheets that you cut to fit the area where you want to put it. Below are images of my current bicycles. I picked up the reflective tape at an auto store. It only costs a few dollars and it gives me greater visibility at night.

Roll your mouse over the images to see what the bikes look like when light hits them at night.