There are a whole bunch of effects that you tend to get when conditions change. Same thing happens in traffic where (say) a set of traffic lights goes out, and suddenly there are fewer accidents. Of course, this is because the lights being out puts everyone on their best behaviour and the accident rate goes back up as people get used to it.

Thank you that is the guy who I was thinking of - its a long time since I did my production engineering courses.

Hawthorne was a place.

I could also be the change itself and not the final state that causes it, or not?

That's one of the conclusions. People assumed the changes lighting meant they were being monitored or watched in some way, so they performed better.