You computer is probably only going into a high level sleep mode, effectively idle, and sensitive mice have a habit of breaking that state with the simplest vibration on the desk. You'll probably need to change the sleep state to a lower level, such as S3 or S5 - this is done through the BIOS, or you could hibernate as well. This will drop the PC into a much lower power state and disable external devices, typically, you have to use the system power button to turn it back on. Some BIOS's will have options for power-on, such as wake-on-lan, wake-on-keyboard, wake-on-device, etc. You may get some options in your OS, but typically, it's the motherboard that has the final say.