The study examined the consequences of working conditions on the previous day on cognitive performance the following day. It also addressed the issue of whether this relationship was mediated by sleep and whether it differed as a function of age. The data were taken from the VISAT study (aging, health and work) and concerned the participant's general work schedule, general sleep quality, working conditions on the previous day (content, duration, load and schedule), subsequent sleep length and quality, and cognitive performance. Results showed that both physical activity and working before 6 am or after 10 pm on the previous day were significantly associated with poorer cognitive performance. Significant effects of working conditions on the previous day were also observed on subsequent sleep, but these effects did not mediate the relationship between working conditions and cognitive performance. The observed effect on cognitive performance of atypical work hours on the previous day was similar for all ages, probably because of the healthy worker effect.