Why does my wireless internet work better at night?

My internet during the daytime is shoty at best problem opening even the simplest websites , even after checking all connection issues putting the laptop feet away from the router even using tcp/ip optimizer but when the sun goes down my connection speed JUMPS .... this problem only started recently no changes have been made to the computer and before now the internet is normally over 20Mb/s ... wired connections are at 20Mb/s all day ... and i know it is not the channel the WIFI is on .... Any ideas?