Using data from a Japanese time use survey, we show a noteworthy increase in the share of employees working at unusual hours (late night and early morning) over a period of a decade since the mid-1990s. When controlling for changes in hours worked, however, we find that the notable increase in the fraction of people at work at unusual hours was for low-income nonregular employees (part-time, temporary and contract workers) while relatively higher income regular employees' work timing remains stable. These observations imply that there is a trend of diversification of work timing in Japan between regular and nonregular employees. A possible explanation is that the increase in the average hours worked per weekday by regular employees, possibly because of the spread of the 5-day workweek since the 1990s, increased services and goods demand at unusual hours as they returned home. An Oaxaca-Blinder type decomposition suggests that such an increase in the average hours worked by regular employees explains partially the rise in the employment rate of nonregular employees at unusual times.
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