The sleep function of MacOS X is dubious at best. Sleep was designed for Powerbooks to conserve battery life. but some idiot thought that it would work well on non-powerbooks.
Well, it has caused mucho troubles for people leaving their computers on and letting them nod off. This and other forums are repleat with user woes after waking up a sleeping MAC.
Don't allow you system to sleep if you keep any software apps open. It just causes too many problems and is counterproductive. I do allow my monitor to turn off after a specfied period of time but no spinning down hard drives or the like.