It's a natural phenomenon that our senses are finely tuned at night time, it's a time where in nature we would be more vulnerable to attack. Some people are more sensitive to this than others. I experience similar, especially at times when I'm feeling a bit stresses or isolated - I believe it's all in the mind and that your sister just needs to learn to re-assure herself of this.
It might help if she turns this into something creative, perhaps memorising what she's seen then drawing/painting it.