On my Windows Vista workstation, when I lock the computer for the night.(I Lock, not Log off). When I unlock in the morning all network connections were dropped and re-enabled. I can lock the computer for short periods during the day and not have this issue. I do not know if it is ocurring on lock or on un-lock. Any help would be great Thanks!!
Could this be your router failing or wireless connections getting dropped in the night? Could it also be that you have automatic updates enabled, and Windows is going ahead and installing those and for some reason you are consistently getting new Network drivers which it installs during the night?