Norman thinks that the brain function of students suffers when they receive less sleep. he predicts that students who get less sleep before an exam will receive lower test grades. to investigate his idea, he tests two groups of chemistry students. group 1 receives four hours of sleep, while group 2 receives the recommended eight hours of sleep the night before the exam. both groups consume the same foods before testing and take the same exam in the same room. the room has a constant temperature and level of light for all students. what is the independent variable in norman's investigation?