I found it rather interesting reading about the Hawthorne Effect:
'A term referring to the tendency of some people to work harder and perform better when they are participants in an experiment. Individuals may change their behavior due to the attention they are receiving from researchers rather than because of any manipulation of independent variables'
Accessed from: http://psychology.about.com/od/hindex/g/def_hawthorn.htm
...and even more interesting to find out that when the original data were re-examined, it was found that the lighting conditions at the factory were always changed on a Sunday and so productivity on a Saturday was compared to the new conditions on a Monday. Further investigation showed that whether lighting was changed or not, productivity always went up on a Monday as it was the start of the week.
Accessed from: http://www.economist.com/node/13788427?story_id=13788427