Time of day and naturalistic prospective memory

Fatigue and circadian rhythms have been proposed as explanations of time-of-day effects on retrospective memory. Both explanations predict poor morning performance, a midday performance peak, and then declining performance and poorest recall in the evening. Whereas this pattern has been found in research on retrospective memory using ordered serial recall, in the single relevant study on prospective memory, performance was found to be highest in early morning, followed by a midday decline with no additional declines. Using post hoc analyses, we investigated older adults' prospective memory throughout the day. Data were taken from two studies we had conducted previously and one recently completed experiment. In each of the three experiments, we examined simulated medication and appointment adherence over a 13-day period and found prospective memory to be better in the morning than at midday. In two experiments, we found no further decline after midday, and in the third experiment, performance actually increased in the evening compared with midday. These post hoc analyses provide preliminary evidence that factors different from or at least in addition to fatigue and circadian rhythms produce time-of-day effects on prospective memory, indicating the need for more programmatic research in the future. One explanation for these findings is that attentional capacity devoted to prospective-memory tasks varies inversely as a function of activity level during different times of the day.