When Sarah thought that she heard a strange noise, the sympathetic nervous system of her triggered the 'fight-or-flight' response. This response is triggered in stressful situations. However, when the perceived threat was proved to be false, the parasympathetic nervous system starting slowing down her bodily functions to a normal condition. The parasympathetic nervous system triggers the 'relax-and-digest' response.
Hence, the answer is parasympathetic nervous system'.
