Many competing theories have been advanced to discover the possible connections between sleep and learning in humans. One theory is that sleep consolidates and optimizes the layout of memories, though recent evidence suggests this may be restricted to implicit procedural memories.
Popular sayings can reflect the notion that remolded memories produce new creative associations in the morning, and that performance often improves after a time-interval that includes sleep. Many studies demonstrate that a healthy sleep produces a significant learning-dependent performance boost. Healthy sleep must include the appropriate sequence and proportion of NREM and REM phases, which play different roles in the memory consolidation-optimization process. In motor skill learning, an interval of sleep may be critical for the expression of performance gains; without sleep these gains will be delayed (Korman et al., 2003). However, several studies show that, in some conditions, time after training, even without sleep, may suffice for attaining significant performance boosts (Roth Ari-Even et al., 2005).
In a study, procedural memories have been shown to benefit from sleep (Walker et al., 2002, as cited in Walker, 2009). Subjects were tested using a tapping task, where they used their fingers to tap a specific sequence of numbers on a keyboard, and their performances were measured by accuracy and speed. This finger-tapping task was used to simulate learning a motor skill. The first group was tested, retested 12 hours later while awake, and finally tested another 12 hours later with sleep in between. The other group was tested, retested 12 hours later with sleep in between, and then retested 12 hours later while awake. The results showed that in both groups, there was only a slight improvement after a 12 hour wake session, but a significant increase in performance after each group slept. This study gives evidence that sleep is a significant factor in consolidating motor skill procedural memories.
Declarative memory has also been shown to benefit from sleep, but not in the same way as procedural memory. A study was conducted where the subjects learned word pairs, and the results showed that sleep not only prevents the decay of memory, but also actively fixates declarative memories (Ellenbogen et al., 2006, as cited in Walker, 2009). Two of the groups learned word pairs, then either slept or stayed awake, and were tested again. The other two groups did the same thing, except they also learned interference pairs right before being retested to try and disrupt the previously learned word pairs. The results showed that sleep was of some help in retaining the word pair associations, while against the interference pair, sleep helped significantly.
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