I think that leaving the PC "asleep" or in the stand by mode is the same as shutting it off. It would be better to leave in the "always active" mode so that the hard drive wont have to wake up and recover each time you move your mouse. That will strain it and shorten its life as well.
I have gone through about 5 PC's now, and usually because out dated. / With the newer computers I really believe its best to leave PC on and use the "stand-by" or sleep mode when not using it for longer than 3hours. The only time I turn off, is if I am not going to be using it for longer than 48 hours, or for a Update-restart. Not good for the hard drive to shut it off and on/ will shorten life. / hope this helps.