It is likely that cognitive dysfunction at nonoptimal times of day is related to a general decrease in physiological arousal or alertness, because self-reported time-of-day preferences correspond to cyclical fluctuations in physiological measures, including body temperature, skin conductance, and heart rate. If time-of-day- effects are simply due to fluctuations in physiological energy, then one might expect that the performance of older adults during the afternoon might be facilitated by substances that increase arousal