The present study aimed to demonstrate the prospective association of workaholism (i.e., working excessively hard in a compulsive fashion) with sleep quality among Japanese employees. A prospective survey was conducted among registered monitors of a survey company in Japan at two points separated by a 7-month interval. The data were collected using a self-administrated questionnaire that included scales on workaholism, sleep quality, job characteristics, and demographic characteristics. The mean age of participants at baseline was 45.9 years and 48.7% of participants were women.