learning. Together these studies show that newly learned memories are reactivated during sleep and through this process new memory traces are consolidated. In addition, researchers have identified three types of sleep (SWS, sleep spindle and REM) in which declarative memory is consolidated. Slow-wave sleep, often referred to as deep sleep, plays the most important role in consolidation of declarative memory and there is a large amount of evidence to support this claim. One study found that the first 3.5 hours of sleep offer the greatest performance enhancement on memory recall tasks because the first couple of hours are dominated by SWS. Additional hours of sleep do not add to the initial level of performance. Thus this study suggests that full sleep may not be important for optimal performance of memory. Another study shows that people who experience SWS during the first half of their sleep cycle compared to subjects who did not, showed better recall of information. However this is not the case for subjects who were tested for the second half of their sleep cycle, as they experience less SWS. Another key piece of evidence regarding SWS's involvement in declarative memory consolidation is a finding that people with pathological conditions of sleep, such as insomnia, exhibit both reduction in Slow-Wave Sleep and also have impaired consolidation of declarative memory during sleep. Another study found that middle aged people compared to young group had a worse retrieval of memories. This in turn indicated that SWS is associated with poor declarative memory consolidation but not with age itself. Some researchers suggest that sleep spindle, a burst of brain activity occurring during stage 2 sleep, plays a role in boosting consolidation of declarative memories. Critics point out that spindle activity is positively correlated with intelligence. In contrast, Schabus and Gruber point out that sleep spindle activity only relates to performance on newly learned memories and not to absolute performance. This supports the hypothesis that sleep spindle helps to consolidate recent memory traces but not memory performance in general. The relationship between sleep spindles and declarative memory consolidation is not yet fully understood. There is a relatively small body of evidence that supports the idea that REM sleep helps consolidate highly emotional declarative memories. For instance Wagner, et al. compared memory retention for emotional versus neutral text over two instances; early sleep that is dominated by SWS and late sleep that is dominated