Thanks reol. but I think the only thing permanently connected to my pc that ever goes to sleep is me 
In fact, when the audio interface is not connected, there are only my external audio drive (which is not asleep!) a usb for my waves licences and the Steinberg licence key plugged into the usb ports
I just re-installed the "ASUS AISuite III"to my PC (I removed it when I was doing driver updates several weeks ago) and it seems to have made the problem a lot more frequent…may be coincidence though.