A paper about decision fatigue was featured recently in the Economist. Its results were fascinating (and also a little terrifying). 

The paper—written by Cambridge University researchers Tobias Baer and Simone Schnall and available online here—supports the hypothesis that the quality of human decisions decreases when we become tired. The study found that, in the case of 26,501 complex restructure loan applications made over the course of one month at a major bank, a significant dip in the approval of loans was visible around midday (i.e. before the employees took lunch).

But that's not the most mind-blowing part. 

The researchers then went on to show, using additional credit collection data from the same bank, that had there been no dip the bank might have made an additional $509,023 in revenue. 

Quantifying decision fatigue like this, as the researchers have, is pretty remarkable. But it does worry you reading it, though. And not just about the cost of fatigue in the financial factor and how that impacts society, but also about cases in sectors where the result of decision fatigue is not realised (at least directly) in financial terms—for instance, patient outcomes in health care. 

Anyway, I highly recommend giving it a read: https://royalsocietypublishing.org/doi/10.1098/rsos.201059

There's also a shorter Cambridge University summary here.

And here's the Economist article (£).