Use the Screen Filter application which
dulls the screen thereby reducing its brightness. I highly recommend not reading
in darkness. But if you absolutely have to check mails or Reply to SMS near
your bed and if you find the screen brightness too disturbing, try using this
App "Screen Filter". Like I already said, don't read in the Dark! You
are better off with that. Keep your eyes safe and Healthy!!! DO not use your phone in the dark.
In windows you can search for keyboard from start menu and open keyboard settings. Or you can go to Settings > Accessibility > Keyboard. In the keyboard settings window, toggle the switch that says "Use the Print screen button to open screen snipping". Hereafter whenever you press print screen button, you will be able to make a selection on the screen for a screenshot.