It is probably not a setting as such, but more likely a particular glitch that has been around since at least 10.3. Sometimes, for some reason, the AirPort card no longer notices that the computer went to sleep, so when it wakes up, the card doesn't know that it has to search for the network again.
There are a number of things people have tried to fix this, many detailed in the Networking forum. Read
this post and see if that helps you out.