I've been having unpredictable network problems since 15.10 - some I've mentioned here. Some of this could be happening because I too put my computer to sleep when it's not going to be used in a while.
This command should work:
sudo service network-manager restart
-- or --
sudo service NetworkManager restart
It's far from a solution, just a workaround that kicks it back into place, instead of restarting the entire computer, where it seems the networking is a bit buggy in 15.10.