Yes, the light is always on (unless they've recently changed it) while the reader is in use, but you can turn it all the way down. When on standby the light is off. The light does have an effect on battery life, how much depends at least somewhat on how high you're setting the light. It doesn't need to be turned up all that high most of the time. Another way to save on battery is to put the device in "airplane mode" when you don't need wireless. The product page for the Paperwhite explains quite a bit about the device
http://www.amazon.com/dp/B00AWH595M