my insomniac pc
 

I've just it built my PC a couple of weeks ago (my first time), it's alright and it works fine. But one thing i noticed, is that whenever i try to put it in "sleep mode" my CPU doesn't sleep like it should. The monitor will sleep, but the CPU and other fans will keep going. Then, when i try to wake it up (by pressing some keys/moving the mouse), it reboots itself instead of displaying the desktop.

What should i do to resolve this?