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At first glance, sleep seems like a passive state, characterized by reduced responsiveness, relative inactivity, lack of consciousness and a lying posture with closed eyes. But not everything is as it seems. The results of sleep deprivation studies tell us that the loss of only one night's can impair the immune system, cause increased heart rate variability, and with many more physiological and psychological effect it causes cognitive impairment and decreased memory performance. In the field of sleep and memory research, the idea, that sleep plays an active role in the off-line consolidation of memories is getting more and more pronounced [3]. The acquirement of new information is an incredibly fast process (takes milliseconds), which leads to the creation of a new memory element. But this initial state of a memory is very instable and vulnerable, it can be easily manipulated by external stimuli (etc. interference) or even worse, it can disappear. For new memories to get stronger, and to be transferred to the long term memory system, they need to go through the so called memory consolidation processes [9]. During memory consolidation the initial memory becomes stabilized and more resistant to interference, which can be caused by other (external or internal) competing and disturbing stimuli [4]. Theories supporting the active role of sleep claim that not merely reduced interference takes place in the background, but they suppose active sleep specific processes potentiating these off-line consolidation processes [6]. One of these processes is the spontaneous reactivation of memories during sleep, along which the relevant memories are “replayed” and strengthened. The future relevance of memories can be manipulated experimentally (etc. instruction, evoked intention to forget, reward) [5].These manipulations give us the opportunity to compare the consolidation of relevant and irrelevant memories.
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