Adolescents and young adults continue to develop rapidly, especially in their high cognitive functions such as working memory. Sleep and regular exercise have been shown to affect brain and cognitive functions, and we investigated the interplay between these two factors on working memory. One hundred participants completed a sleep-exercise log, wore an acti-watch for five days and performed a working memory task on the sixth day. Regular exercisers were found to have significantly faster response time on the working memory task than non-regular exercisers, and such effect was moderated by actigraphy-measured total sleep time. In other words, regular exercisers had faster reaction time than non-regular exercisers only when they had sufficient sleep (>6.5hours). Our findings showed that both healthy sleep and regular exercise habits should be promoted among adolescents and young adults to optimize cognitive development.