Some Nook users are a bit confused about the function of turning the device on or off. When you press the button on the top of the unit, it just puts the Nook into sleep mode. The screensaver is then displayed on the screen.
Although sleep mode allows you to quickly access the device when you need it, it still uses a bit of battery power. To turn the Nook completely off, press and hold the top button for about 5 seconds until the screen goes blank.