Recognition memory, circadian rhythms, and sleep

One set of 20 subjects learned a list of words early in the morning while another set of 20 learned the same material in the midafternoon. Half of the subjects (n=10) were tested after an intervening four hours of sleep, while the other 10 were tested after four hours of waking activity. Subjects in the afternoon groups performed better on a recognition task than those in the morning groups, while subjects who had intervening periods of sleep between learning and testing performed better than their counterparts who remained awake. Finally, those subjects who learned in the afternoon and slept prior to testing did best on the recognition task, while those who learned in the morning and remained awake did worst. These results are consistent with the notion that both sleep and circadian rhythms play a role in the memory process.