how to get Time Machine backup overnight instead of every hour?

There's a nice looking piece of freeware worth trying called Time Machine Editor which looks like it'll do the trick.

Use TimeMachineEditor. Free. Doesn't say, but it works on Lion too.

TimeMachineEditor is a software for Mac OS X 10.5 Leopard and Mac OS X 10.6 Snow Leopard that lets you change the default one-hour backup interval of Time Machine. You can change the interval or create a more sophisticated scheduling (see screenshot below).

This is useful if you don’t need to backup every hour and don’t want the performance penalty. This is also especially useful if you manipulate lots of data within one hour as you would spend the whole day backing up.