Since time doesn't move in this game, you don't ever have to go to bed to let Nancy sleep. But there is another reason to send Nancy to bed --
-- and that's to change the tide level.
See that sign next to the entrance to the restaurant? It tells you whether the tide is high or low, and there are some tasks where the state of the tide makes a difference.
So if the tide is high and you want it to be low, or vice versa, go up the stairs. When you come downstairs again, the only thing that will have changed is the tide; everything else will be exactly as it was.