How to make it shut automatically off?

Before I got Sansa Clip+ 4GB, I had Sansa Clip, which has the auto shuttin off function. On Clip+ I couldn’t find it, so it happens I forget to turn it off and it stays in stand-by mode for all night and then, of course, the acc. is running down. Using of the sleep mode won’t do.

Is it necessary to install Rockbox to have it (though I don’t know if Rockbox has this function) or there is another way to solve this problem?

Thank You in advance.

Clip+ User Manual

Power Saver: Pages 24 - 25.