Toggle data/wifi when unused, or use an Activator custom activation. Reason is that even though you're on wifi, your data network is still constantly checking to see whether wifi's connection is feasible or not, or it will switch over at a moment's notice. Vise versa, if you're using data with no wifi network, as you're traveling by car or walking, your phone is constantly looking for wifi connections, and will refresh them as you move in and out of range. If you're traveling underground, toggle on Airplane mode such as if you're commuting to work by train for over an hour. Phones are actually really "dumb" yet smart some may say. You're underground, but your phone can't differentiate being underground and having bad signal, so it'll constantly tweak its frequency to try to grab a better signal, and it doesn't stop until you're in range of signal, so that's a bit of a burner, especially if you have data and wifi on at the same time, as the above applies as well.
