Why does the professor mention lighting?

Now listen to part of a conversation between a student and a professor. (P = Professor, S = Student)
P: Yes, but that’s just it. When people know they’re being watched, they act differently. Let me explain how the Hawthorne effect got its name and well, you’ll get the idea. See, there was a manufacturing facility called the Hawthorne plant. And researchers conducted some experiments there to see what conditions made workers the most productive.
S: What sort of conditions?
P: Well, one thing they experimented with was the lights. Were workers more productive with bright lights or dim lights? Well, here’s the thing. Whatever the researchers did, the workers’ productivity increased. When the lighting was improved, productivity went up. When the lighting was dimmed, productivity went up again.
S: That doesn’t make a lot of sense.
P: Exactly! So initially, the experiment was considered a failure. But then, the researchers realized that their own presence had affected the workers’ productivity. The workers knew that the researchers were watching them. And with so much attention on them, the workers felt compelled to work harder.
S: Oh, I guess that really could be an issue with my group’s research…