Welcome aboard RJ12!
Well, whatever the cause of the router - green frog Shizella (as in Cinderella) Syndrome it now disconnects at 1.00 am:
[01:00] *** You have been disconnected. Fri Mar 30 01:00:02 2007.
So that means I am still up at that time to even notice

It isn't unknown for me to still be on line much later if I am not working first thing in the morning.