This is an extract from Amazon.com homepage.
Adjust the Screen Light
The screen light provides the convenient option to read in low light conditions. Kindle Paperwhite's E Ink display provides a crisp and glare-free reading experience with or without the light.
To use the screen light, select while reading.
Slide your finger along the scale to adjust screen brightness.
Tap and hold the - to turn off the light.
Tap and hold + to turn the light on at maximum brightness.
Tip: A lower setting works best for rooms with low lighting, and a higher setting for rooms with bright lighting.
The video is also nice to watch!