A study was created to test the effects of jazz music on people’s sleep patterns. The hypothesis of the experiment was, if people listened to jazz music as they fall asleep, then they will sleep for a longer period of time. For the experiment, 2 groups of people were created. One group was placed in a quiet room, without music where they went to sleep and they were timed on how long they slept. The other group was placed in a room where jazz music played softly as they began to sleep and played throughout the night. As each group awoke, their sleep times were recorded.
What is independent variable in this experiment?