Imagine that you are conducting an experiment to prove that the correlation in the previous question is a causal relationship. You recruit 10 friends, and randomly assign them to two groups: One group will keep their normal schedule and the other group will be excused from their morning class so that they can sleep an extra hour in the morning. Using the same Web application, you ask the subjects to report the number of hours they slept, along with how sleepy they feel three times during the day.

What is a hidden variable that might be at work in this study

Question

Imagine that you are conducting an experiment to prove that the correlation in the previous question is a causal  relationship. You recruit 10 friends, and randomly assign them to two groups: One group will keep their normal schedule  and the other group will be excused from their morning class so that they can sleep an extra hour in the morning. Using  the same Web application, you ask the subjects to report the number of hours they slept, along with how sleepy they  feel three times during the day. 

What is a hidden variable that might be at work in this study