Sleep enhances strategic thinking at the expense of basic procedural skills consolidation

Recent studies show that sleep facilitates the learning of complex cognitive skills. Here, we assess the effect of sleep on performance in an ecological, multi-componential task, which requires subjects to trace on a screen as many words as possible with 16 letters, some of which ("bonuses") multiply the value of letters or words containing them. In a within-subjects design, 23 healthy adults underwent training and retest, with a retention period (approximately 8 hr) spent awake (WK, with training in the morning and retest in the afternoon) or asleep (SL, with training in the evening and retest in the morning). The main performance measure (GLOB) results from the total value of the letters used, the number of words, their length and the strategic use of bonus letters. An additional measure (WORDS, i.e., the proportion of words correctly detected over all detectable words) was also used, mainly reflecting procedural rather than strategic skills. In WK, although GLOB increased at retest, a significant improvement emerged only for WORDS, whereas in SL only GLOB was enhanced. In WK, the GLOB improvement appears to depend on the increase in the number of words detected (GLOB and WORDS improvement measures were positively associated), whereas in SL this association was not observed, indicating a shift to more complex but more rewarding strategies. Our data contribute to the understanding of everyday life learning processes by suggesting that sleep benefits memories of future relevance and promotes preferential consolidation of strategic skills when this is useful to achieve one's goal.