Sleep Microstructure and Memory Function

Several studies investigated the effect of sleep on memory. In humans, arguably the first experimental description of a ben-eficial role of sleep for memory stabiliza-tion was provided in 1924 (1), indicating a protective benefit of sleep in prevent-ing the normal decay-curve of forgetting that develops across time spent awake. Specific stages of sleep appear to be criti-cal for memory consolidation. Memory is commonly divided into a declarative and a non-declarative memory system (2). Declarative memory is defined by memo-ries accessible to conscious recollection, i.e., memories for events in a spatio-tem-poral context (episodic memory) and fact-based information (semantic memory). Procedural memory for skills is the type of non-declarative memory most thoroughly studied with regard to the effects of sleep. Sleep is characterized by the cyclic occur-rence of REM and NREM sleep comprising SWS (sleep stages 3 and 4) and lighter sleep stages 1 and 2. Two hypotheses were pro-posed regarding sleep stages and memory consolidation (3). The dual process the-ory assumes that the specific sleep stages support consolidation of different types of memories. SWS supports declarative memory consolidation whereas REM sleep does so for procedural memories ( 4). The sequential hypothesis, on the other hand, proposes that sleep benefits memory opti-mally through the cyclic succession of both SWS and REM sleep. The original version of this hypothesis assumed that SWS func-tions to weaken non-adaptive memory traces whereas REM sleep re-stores the remaining traces (5). The dual process hypothesis received support mainly based on the early late sleep comparison, i.e., an approach comparing effects of retention intervals covering the first (SWS-rich) or the second (REM sleep-rich) half of nocturnal sleep. SWS-rich early sleep con-sistently found to support consolidation of hippocampus-dependent declarative memories, whereas REM sleep benefited non-declarative types of memory like priming, and memories for visuo-motor skills ( 6, 7). However, this dichotomy does not fit all results. Several non-declarative tasks, like visual texture discrimination, are also supported by SWS whereas REM sleep in some instances seems to benefit aspects of declarative memory (8). More recently, Genzel and colleagues (9) tried to clarify the link between spe-cific sleep stages and different types of memory consolidation by suppressing sleep stages. They deprived subjects once each of REM sleep and SWS, and once let them sleep undisturbed through the night. After each night, the authors tested declarative and procedural memory con-solidation. Although REM sleep and SWS awakenings led to a significant reduction of the respective sleep stages, memory con-solidation remained unaffected. According to the authors, there are two possible explanations for REM deprivation and SWS deprivation not influencing sleep-dependent consolidation of motor tasks: (1) the diminished amount of REM sleep in the REM deprivation condition was still sufficient for sleep-dependent memory consolidation or (2) the memory con-solidation is dependent on stage 2 sleep. Another explanation of the results is that sleep-associated processes contributing to memory consolidation requires analyses of polysomnographic phenomena, like sleep spindles and slow-wave activity (SWA). This aspect is very important also for the implications for the cognitive decline by increasing age (10, 11). Other factors that change with age, such as hormones and neurotransmitters, or hypoxia related to sleep-disordered breathing (12, 13), may also play an important role.