It could be a lot of things, except that you only see it in the morning. Have you checked it at other times of the day? Could it be that your waterbed leaks from underneath, but only when you are laying on it? It could leak from beneath without affecting your sheets.
You should also see whether this spot is close to a pipe for a sink, a tub, a dishwasher, a refrigerator, whatever might be on the other side of the wall. There could be a drain or supply pipe leaking inside a wall, with water running down the wall, and onto your floor.