I think I may have the answer to your questions and concerns.
Sleep mode is, as said previously, a low power mode where the screen, disk activity, and any other peripherals are turned off with the CPU being put in a halted state and the memory preserved.
This COULD reduce lifespan, but I wouldn't worry about it. This sorta thing really doesn't cause major life limiting with computers.
If you are REALLY concerned you could hibernate the computer. This writes all the user memory and important information to the hard disk and turns the computer off. May or may not be a windows only feature however, but if you are using a Macintosh in a professional field, you deserve to be put in a special mental facility.
So in short, wouldn't worry, if you want to worry, use hibernation mode.
But don't change parts when your hibernated. That can ruin windows.