This is what Microsoft says about Sleep Mode;
"Sleep is a power-saving state. Sleep saves all open documents and programs, and allows the computer to quickly resume full-power operation (typically within several seconds) when you want to start working again. Putting your computer to sleep is like pausing a DVD player: the computer immediately stops what it's doing and is ready to start again when you want to resume working".
In Sleep Mode Memory is preserved in RAM so that when you wake the system us again, the system is ready to use as quickly as possible.
Shutdown is none of that. No programs or documents are saved and no power goes to the hardware. RAM, (Memory), is cleared. The system is dead and when you turn it back on the computer has to work through all it's checking and loading routines before you can use it.
Is Sleep Mode safe? Generally Yes, although a power failure may cause problems which would not occur if the system is shut down. RAM does not get cleared and while on newer systems today that is not such a problem, on older systems RAM Memory management was not as good, and that could cause performance problems over time.
Although the system is in a state of "Pause", power is still being used and so the circuits are heated and the fans are working continuously. Whether that causes extra wear and tear might be a factor.
Even if you use Sleep Mode continuously, it might still be a good idea to shutdown on occasion and boot up only when net required, to clear RAM and to give the components a 'rest'.
There is an alternative and that is Hibernation Mode. That is also described in the Microsoft link. Here all open applications and files are saved in a file called hyberfil.sys onto the hard disk. The system is then shut down and when restarted, instead of going through the whole check and load up routine, Windows reads that hyberfil.sys file and restores the system to the state it was before the shutdown.
Does that help?
its has been weeks since i have shutdown, is this safe or can it cause problems? i have not had any problems using this method to date, i am using a desktop with windows 7