Life on a medical roller coaster sucks. As a potentially sane person you do your best to tamp down any part of those 'what if' fears during the day time. This is a big part of learning to cope with a crappy medical diagnosis, especially a cancer one.
But then comes night time. You are trying to sleep and you dream about 'bad things'. All those evil little 'what ifs' come lurking into your head. Then you can't sleep. Then you are cranky because you are tired. And you can't control your brain because you didn't get enough rest.