Sleep has been identified as a state that optimizes the consolidation of newly acquired information in memory, depending on the specific conditions of learning and the timing of sleep. Consolidation during sleep promotes both quantitative and qualitative changes of memory representations. Through specific patterns of neuromodulatory activity and electric field potential oscillations, slow-wave sleep (SWS) and rapid eye movement (REM) sleep support system consolidation and synaptic consolidation, respectively. During SWS, slow oscillations, spindles and ripples - at minimum cholinergic activity - coordinate the re-activation and redistribution of hippocampus-dependent memories to neocortical sites, whereas during REM sleep, local increases in plasticity-related immediate-early gene activity - at high cholinergic and theta activity - might favour the subsequent synaptic consolidation of memories in the cortex.

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The brain encodes huge amounts of information, but only a small fraction is stored for a longer time. There is now compelling evidence that the long-term storage of memories preferentially occurs during sleep. However, the factors mediating the selectivity of sleepassociated memory consolidation are poorly understood. Here, we show that the mere expectancy that a memory will be used in a future test determines whether or not sleep significantly benefits consolidation of this memory. Human subjects learned declarative memories (word paired associates) before retention periods of sleep or wakefulness. Postlearning sleep compared with wakefulness produced a strong improvement at delayed retrieval only if the subjects had been informed about the retrieval test after the learning period. If they had not been informed, retrieval after retention sleep did not differ from that after the wake retention interval. Retention during the wake intervals was not affected by retrieval expectancy. Retrieval expectancy also enhanced sleep-associated consolidation of visuospatial (two-dimensional object location task) and procedural motor memories (finger sequence tapping). Subjects expecting the retrieval displayed a robust increase in slow oscillation activity and sleep spindle count during postlearning slow-wave sleep (SWS). Sleepassociated consolidation of declarative memory was strongly correlated to slow oscillation activity and spindle count, but only if the subjects expected the retrieval test. In conclusion, our work shows that sleep preferentially benefits consolidation of memories that are relevant for future behavior, presumably through a SWS-dependent reprocessing of these memories.

Sleep supports the consolidation of memory in adults. Childhood is a period hallmarked by huge demands of brain plasticity as well as great amounts of efficient sleep. Whether sleep supports memory consolidation in children as in adults is unclear. We compared effects of nocturnal sleep (versus daytime wakefulness) on consolidation of declarative (word-pair associates, two-dimensional [2D] object location), and procedural memories (finger sequence tapping) in 15 children (6-8 yr) and 15 adults. Beneficial effects of sleep on retention of declarative memories were comparable in children and adults. However, opposite to adults, children showed smaller improvement in finger-tapping skill across retention sleep than wakefulness, indicating that sleep-dependent procedural memory consolidation depends on developmental stage.Compelling evidence has been accumulated that sleep supports the consolidation of newly acquired memories in adults (Maquet 2001;Stickgold 2005;Born et al. 2006). Declarative memories benefit particularly from slow wave sleep (SWS), whereas procedural memories benefit particularly from REM sleep (Plihal and Born 1997;Peigneux et al. 2004;Marshall and Born 2007), aside from distinct contributions of non-REM sleep stage 2 to memory consolidation (Gais et al. 2002;Fogel and Smith 2006;Peters et al. 2007). Childhood, compared with adulthood, is characterized not only by distinctly greater amounts of sleep and SWS (Anders et al. 1995;Ohayon et al. 2004) but also by a tremendous extent of brain and behavioral plasticity, determining the child's capability to rapidly acquire huge amounts of facts and to effectively shape skills in response to environmental challenges (Li et al. 2006;Brehmer et al. 2007). However, the role developmental sleep plays for consolidating memory has only been scarcely examined. Restriction of sleep in schoolchildren was shown to be associated to impairments in different cognitive functions (Carskadon et al. 1981;Randazzo et al. 1998;Steenari et al. 2003). Animal studies provided considerable evidence that developmental sleep, like sleep in adults, is crucially involved in brain plasticity (for review, see Dang-Vu et al. 2006). However, in a recent human study (Fischer et al. 2007), children aged 7-11 yr, in contrast to adults, showed impaired rather than improved implicit sequence knowledge in a procedural serial reaction-time task when training was followed by periods of sleep, pointing toward differential dynamics of sleep-dependent consolidation of procedural memories during development.In the present study, we dissociated effects of post-learning sleep on procedural and declarative types of memories in 15 healthy children (age 6-8 yr, mean ‫ע‬ SEM: 7.5 ‫ע‬ 0.16 yr; 9 females, 6 males) and 15 healthy adults (26.5 ‫ע‬ 1.3 yr; 13 females, 2 males). The study was approved by the local ethics committee, and informed consent was obtained by participants and the children's parents. All subjects had normal sleep and were adapted to standard polysomnographic recordings (obtained by a...

Memory consolidation is a dynamic process. Reconsolidation theory assumes that reactivation during wakefulness transiently destabilizes memories, requiring them to reconsolidate in order to persist. Memory reactivation also occurs during slow-wave sleep (SWS) and is assumed to underlie the consolidating effect of sleep. Here, we tested whether the same principle of transient destabilization applies to memory reactivation during SWS. We reactivated memories in humans by presenting associated odor cues either during SWS or wakefulness. Reactivation was followed by an interference task to probe memory stability. As we expected, reactivation during waking destabilized memories. In contrast, reactivation during SWS immediately stabilized memories, thereby directly increasing their resistance to interference. Functional magnetic resonance imaging revealed that reactivation during SWS mainly activated hippocampal and posterior cortical regions, whereas reactivation during wakefulness primarily activated prefrontal cortical areas. Our results show that reactivation of memory serves distinct functions depending on the brain state of wakefulness or sleep.