I know this will sound basic but make sure your light sensor at the top of the device is clean and clear. If you shadow it while you're waking the device or it's dirty, which it doesn't take a whole lot to be dirty enough to have an effect, the device will think it's in low light and delay or prevent the screen from reaching the appropriate brightness for the lighting conditions. To prove or disprove this go in and manually set the brightness, turn off auto, to a higher level and see if you experience this issue. If the device illuminates correctly look into what could be affecting your light sensor. i.e. Is the way you're holding it that is creating a shadow, partially covering it, touching it, is it dirty, are you waking it in a dimly lit room, etc etc. Most times if this sensor goes out it will either fail on or off and won't go back and forth, so you restarting the device and achieving a positive result makes me feel that the issue is correctable and not in the device... Same goes for the back light, unless there is a short in the device from a fall or other impact, the backlight will simply not work. It won't be intermittant your screen will just be dark all of the time, and restarting it won't make a difference. Other tests you can do to ensure your sensor is working is simply shine a light on it or move it closer to a light source when it seems to not want to illuminate, or when it is illuminated cover the sensor and see that it dims the screen.
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