The goal of this study was to investigate the impact of circadian arousal on prospective memory performance as a function of age. We tested a younger (18– 34 years) and an older group (56– 95 years) of participants on- and off-peak with regard to their circadian arousal patterns in a computer-based laboratory experiment. For the prospective memory task, participants had to press a particular key whenever specific target words appeared in an ongoing concreteness-judgment task. The results showed that prospective memory performance was better on- than off-peak in younger but not older participants. Younger participants consistently outperformed older participants in all conditions. We conclude that prospective remembering underlies time-of-day effects which most likely reflect controlled processes.