A researcher is investigating alertness. She asked 20 subjects who habitually had either 6, 7, 8, or 9 hours of sleep every night to complete the Stanford Sleepiness Scale (SSS). The SSS is a measure of alertness and simply requires the subject to rate his or her level of alertness by responding with a number between 1 (very alert) and 7 (struggling to stay awake). Each subject did this in the morning, the afternoon, the evening, and at night. The subject was also asked to self-identify as either a night-person or a morning-person. It is safe to assume that none of the subjects have unusual hours, such as getting up at 10 at night.

List all of the variables in the experiment.