Fabrizio Sanna and colleagues at the University of Cagliari in Italy injected oxytocin into particular parts of the brains of rats and induced yawning, or blocked the effect by injecting an oxytocin-blocking chemical first.
This could be interpreted as a subtler version of the Hawthorne effect, if you believe that workers were aware that changes in artificial light were induced by the experimenters, whereas natural light was changing on its own.