Changes in working hours may have two contrasting effects on productivity: a “fatigue effect” and a “learning effect.” An increase in working hours may lead to the accumulation of workers' proficiency and skill in his or her job, while it may cause the level of workers' fatigue to increase. Our estimation results find multiple thresholds in the linkage between working time and productivity, supporting the existence of learning effect as well as fatigue effect from working time extension. The results of our study provide some implications for the recent discussion on reform of the working time system: for example, (i) the overall effects of changes in working time depend upon a variety of factors, such as the initial level of working hours and the features of the industry concerned; and (ii) job sharing may induce employers to substitute part‐timers for full‐time workers, thereby hindering workers' accumulation of skills. (JEL J24, D24, D92, O47)