Several companies in Japan introduced early working conditions (including recommendations on early morning work and prohibitions on nighttime overtime work) to decrease the number of long working hours at night. Nevertheless, individuals possess their own chronotype, i.e., their behavioral timing preference-be it morning or evening-that is associated with worker health. The purpose of this study was to investigate the influence of chronotype and working conditions on sleep and health related quality of life (HRQOL) using 126 daytime office workers who were classified as morning or evening type by their Morningness-Eveningness Questionnaire scores. We then compared morning and evening type workers' sleep variables (sleep onset/offset time and total sleep time), sleep quality (using the Japanese version of the Pittsburgh Sleep Quality Index), and HRQOL scores. Additionally, we compared the same sleep variables, sleep quality, and HRQOL scores of each chronotype category of worker under early and normal working conditions. As the results, evening type workers had late sleep onset/offset time, poor sleep quality, and low HRQOL (role-social component) compared to morning type workers. Furthermore, the evening type workers under early working conditions had earlier sleep onset/offset time and poorer sleep quality compared to those workers under normal working conditions. These results suggest that evening type workers in general have poor sleep and low HRQOL and those same workers under early working conditions, in particular, are associated with poor sleep quality. Therefore, in order to optimize worker health, we suggest that working conditions should be taken account of individual chronotypes.
