Why did Rosy study until very late at night?

Soon it would be the holidays, but before that, there was the end of year exams. All the students had been working hard for some time. If they didn’t pass, they would have to take them again in September. There were usually a few who failed, but Rosy was determined not to be one of them. She had worked hard all year, but just before the exams she was working so hard that her sister Betty was worried about her. She was staying up too late. The night before the first exam, Betty insisted that she should have an early night and take a sleeping pill. She promised to wake her in the morning.
As she was falling asleep, Rosy was worried that she might oversleep. Her mind kept jumping from subject to subject. At last, with the help of the pill, she fell asleep. In no time at all, she was sitting in the examination hall, looking at the paper. She couldn’t answer any of the question. Everyone round her was writing pages and pages. However hard she thought, she couldn’t find anything to write about. She kept looking at her watch. Time was running out. There was only an hour left. She started one question, wrote two sentences, gave up and tried another one. With only half an hour left she wrote another two sentences. By this time she was so worried that she started crying. Her whole body shook. It shook so much that it woke her up. She was still in bed and it had all been a terrible dream. A minute later, Betty called her name.