An ultra short episode of sleep is sufficient to promote declarative memory performance

Various studies have demonstrated that a night of sleep has a beneficial effect on the retention of previously acquired declarative material. In two experiments, we addressed the question of whether this effect extends to daytime naps. In the first experiment we assessed free recall of a list of 30 words after a 60 min retention interval that was either filled with daytime napping or waking activity. Memory performance was significantly enhanced after napping as opposed to waking but was not correlated with time spent in slow wave sleep or total sleep time within the napping condition. The second experiment was designed to clarify the role of total sleep time and therefore included an additional third group, which was allowed to nap for no longer than 6 min on average. In comparing word recall after conditions of no napping (waking), short napping, and long napping, we found superior recall for both nap conditions in contrast to waking as well as for long naps in contrast to short naps. These results demonstrate that even an ultra short period of sleep is sufficient to enhance memory processing. We suggest that the mere onset of sleep may initiate active processes of consolidation which - once triggered - remain effective even if sleep is terminated shortly thereafter.