The USA has been following a doleful British trend of late - postal voting, or as they call it "absentee voting". I've recently seen figures that the normal proportion of absentee votes is about 5%, that 2004 was about 10%, but that it's expected to be 25% or more this time.
The rules for postal votes in the UK are that they have to be received by the Returning Officer by the close of poll - normally 10pm on Polling Day.
The rules in for absentee votes in much of the US are that they have to be posted on or before Election Day. This is a unversal rule for military voters using the Defense Department postal system (which includes their families).
This means that there are going to be literally millions of votes that are going to take days - maybe even a couple of weeks - to get to the electoral officers. Given that the American system is to make the official declaration only after counting all the votes (normally several weeks after Election Day), my prediction is that it will be impossible to be sure who will win in many races overnight. Unless the Democrats get a landslide of 1994 (US) or 1997 (UK) proportions, there is no way that control of the House will be settled overnight.
Anyone planning on staying up might want to reconsider.