The authors demonstrate that people discount delayed outcomes as a result of perceived changes over time in supplies of slack. Slack is the perceived surplus of a given resource available to complete a focal task. The present research shows that, in general, people expect slack for time to be greater in the future than in the present. Typically, this expectation of growth of slack in the future is more pronounced for time than for money. In 7 experiments, the authors demonstrate that systematic temporal shifts of perceived slack determine the extent and the pattern of delay discounting, including hyperbolic discounting. They use this framework to explain differential propensity to delay investments and receipts of time and money.
Zauberman, G., &Lynch, J. G., Jr. (2005). Resource Slack and Propensity to Discount Delayed Investments of Time Versus Money. Journal of Experimental Psychology: General, 134(1), 23-37.