Chronopsychology has shown that human mental efficiency fluctuates rhythmically in time, and that the periodicity of these fluctuations varies. Apart from ultradian rhythms, circadian rhythms have been studied the most frequently, particularly in regards to attentional efficiency and memorization. However, the data gathered so far are highly inconsistent and mental efficiency rhythms seem to be modulated by numerous so-called “masking” factors. Instead of using the conventional group methodology, we tested the intraindividual stability of cognitive performance in the course of the day. Attentional capacities and memory were measured for 10 weeks at 9 a.m. and at 4 p.m., always on the same weekday. For certain tasks, the subjects consistently exhibited greater efficiency either in the morning or in the afternoon. The tasks in question were not the same for all subjects, but the fact that systematic fluctuations in performance were found in the same subject shows that the temporal factor cannot be ignored.