This article reviews research that suggests people, when faced with a difficult future task, actually work harder on current tasks. The researchers labelled this tendency as the "Get Ready" mindset, believing that the the brain is preparing for the challenge of the future task by gearing itself up via increased effort on current tasks. From the article:
In a series of five studies, the authors observed that the more difficult a future task was expected to be, the harder consumers worked on a current task. "For example, consumers consulted more information on a web page when they were asked to evaluate a new soft drink when they expected that they would later on have to work on a difficult and demanding task," write the authors. Other participants were better able to come up with weight loss ideas when they believed they would have to work hard on a future job.
The authors titled the phenomenon the "get ready mindset." "People seem to prepare themselves mentally for upcoming tasks, but in doing so, the resources that are freed up for the future task carry over to current tasks," the authors explain. "We found consistent evidence that if the mind gets ready to perform later, it is set to go now."
Basically, people will, in general, work harder overall when a difficult task lies ahead. I found this to be an interesting observation on motivation and effort, and I recommend the whole article.