You might love reading from your tablet or mobile phone before going to sleep, but the person with whom you share a bed might be less than enamoured by the light, which can be as bright as that from a lamp. To avoid disturbing a partner's sleep - to say nothing of a person trying to nap next to you on a train or plane - consider inverting the display's contrast, which will turn all the white areas to black and vice versa, significantly reducing the device's light output, suggests German computer magazine c't.
Make the switch for an iPad or iPhone by going to Settings/General/Accessibility and then picking Invert Colours. With an Android device, go to Settings/General and then pick the Negative Colours option.
But this step is only recommended for surfing, reading or writing, not watching movies or viewing pictures, because of the effect reversing contrast will have on most images, inverting all colours, not just black and white.
If you want to watch a movie without disturbing a neighbour, just set the screen brightness as low as possible.