I estimate the causal effect of working hour reduction on the workers’lifestyle and working conditions. I exploit a natural experiment arising from thetransition to the 40-hour workweek system in Korea. The 40-hour workweeksystem had gradually been adopted over the years at different times byestablishment size. Using difference in difference instrumental variable(DD-IV) estimation method and Korean Working Conditions Survey(KWCS)data, I find that a reduction in working hours significantly improves theworkers’ work-life balance and satisfaction with working condition, while itworsens rest-autonomy. I also find that smoking, drinking and working speedare not significantly affected.