Along with the new iOS 5, there are loads of new features which may be small but quite useful. One of them is night reading mode. When you activate the night-time reading mode from the fonts menu in iBooks, the default white-on-black color scheme is reversed – background goes black while the text color turns white thus causing less strain to your eyes in the dark. The Instapaper app too offers a similar dark theme for your late-night reading sessions.
On your iOS device, tap the Settings icon and choose General –> Accessibility –> Triple-click Home and and set it to “Toggle White on Black.”
Press the Home button to exit Settings and launch any reading app. Triple-click the Home button on your iOS device in quick succession and it should enable white-on-black effect quite similar to iBooks Night mode. Triple-click again to switch to normal mode. Simple!
Also, iBooks have this feature in more UI friendly manner to make reading easier in night mode.