A scientist was interested in knowing how the amount of sleep a student gets the night before an exam affects the score that a student gets on the exam. He divided his subjects (students) into 3 groups. Group A took the exam after sleeping 6 hours. Group B took the test after sleeping 8 hours (normal amount). Group C took the test after sleeping 10 hours. He then looked at the test scores to see if there was any difference between the 3 groups. What is the independent variable in this experiment?