The principal at Maplewood High School observed that students' scores on science exams were lower this year compared to previous years. This year, all students attended science classes during the second period of the school day; in previous years, they had science classes during the last period of the day. Believing that students perform better on science exams when they are more alert, the principal concluded that test scores would improve if science classes were moved to the end of the day. The principal’s reasoning relies on which of the following assumptions?