Thanks Mathew, your post inspired me.
It also works for me with I remove the mouse dongle, but obviously that's not so practical.
I looked at the mouse settings in the device manager and Allow this device to wake the computer was already unchecked, so I searched again in Bing and found the following command:
powercfg -devicequery wake_armed (run as admin)
This showed me 2 USB Root Routers.
I went into the device manager and unchecked Allow this device to wake the computer for both of them and even after a reboot it seems to work.
My laptop now sleeps like a baby