If it's less than 50% humidity in the bedroom while the heat is set to run and it's night time turn on an outlet that the humidifier is plugged into. Don't do it if mode is "Away".
Not that it will ever happen...but if the mode changes while it's on...it will never turn off until the mode changes back. I've gotten burned by restrictions before. Again it's unlikely in your case. I think I would consider putting mode into your conditions. Mode = Day/Evening and Night. That way if it's false...it turns off.
Good point, the rule will never be evaluated to return false using it as a restriction, it should be a condition!