The morning after she had that discussion with Kou, she felt terrible, anxious, and frightened. She woke up feeling sick and nauseous.
Sumire knew what the main cause was, and it was another nightmare that led to this situation.
Others have defined dreams as a figment of one's imaginations; what do others say about nightmares? They say the same thing. It is all made up or depends on the individual's current mood. Nobody, however, would ever guess that it has something to do with an individual's past or physiological/mental state.
Her nightmares came like strange waves. Sometimes she would have them every day and get used to it. But there were times when it would disappear for an extended time and come in short waves. Those short waves, however, were more painful than the everyday ones.