Recognition memory, sleep and circadian rhythms

In Experiment 1, subjects learned a list of words and were tested for recognition of them 24 hours later. For the delayed sleep group, learning was followed by a period of wakefulness; for the immediate sleep group, it was followed by a period of sleep. Retention was significantly better for the immediate sleep group, consistent with the notion that consolidation is enhanced by the interpolation of sleep shortly after learning. In Experiment 2, subjects were tested approximately eight hours after learning. The normal waking group learned and was tested after a period of daytime weakfulness; the normal sleep group learned and was tested after a period of nighttime sleep; and the sleep deprivation group learned and was tested after a period of nighttime wakefulness. Retention for the normal sleep group was superior to that of the normal waking group, thus extending the commonly observed effect of sleep on memory to the domain of recognition memory. However, we found that retention was not better for the normal sleep group than for the sleep deprivation group suggesting that the effect of sleep on memory may be partially due to circadian rhythms.