Nightmare - does the target wake up, or have fitful dreams all night?

[RangerWickett]

When a person has nightmare cast against them, do they take the damage and wake up immediately, or do they take the damage and remain gripped by the horrible visions? What if they make their save? I suppose you might actually want to wake up and warn your buddies if you just had a horrible nightmare (but, y'know, not that horrible, because it didn't scare me).