It could be one of two issues. First of all, make sure your auto-brightness is turned on. The Kindle uses an ambient light sensor and with auto-brightness on, the device can modify the screen to the appropriate light levels. You simply need to turn it on with the button at the bottom of the screen. The other issue is that your surroundings are affecting it negatively. Having a bright screen with no ambient light in the night can amplify the problem. Turning a light on or moving to a better location can help change the lighting gradient.