Dr. Sanchez is a behavioral psychologist who studies learning and memory in rats. Because rats are most active at night, she asks the animal resource facility to reverse their light–dark cycle to accommodate her experiments. However, the animal facility is reluctant to disrupt the rats’ current _____, which lasts for "about a day."

As the exercise suggests, the animal facility is reluctant to disrupt the rats’ current Circadian rhythm. This process is both natural and internal of every individual; it regulates the sleep cycle, repeating around every 24 hours, just as the exercise introduced. Therefore, what this behavioral psychologists is trying to do is to disrupt this cycle, which would cause the reluctant effect out of the rat, which is most active at night.