A group of five people worked continuously for 4 hours grading papers. Every 20 minutes they were each asked to estimate "the satisfyingness or tolerability or zest or interest in the work" on a scale of 0 to 10, where 0 meant extreme distaste and 10 extreme pleasure. This shows the average satisfaction of the five as a function of the 20-minute time interval.
Data from "The Curve of Work and the Curve of Satisfyingness" by E.L. Thorndike, Journal of Applied Psychology, 1917, Vol 1, pages 265-267, as reported in "Quantitative Methods in Psychology" by D. Lewis, McGraw-Hill, New York, 1960.