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 (2002: 68).