In another study, published in 2002, Stickgold, Harvard psychologist Matthew Walker, and colleagues asked subjects to type the sequence 4-1-3-2-4 as quickly and accurately as possible. Those who were retested later in the day showed small improvements. But those who returned to the keyboard the following day could type the numbers roughly 20 percent faster. (Their gain was correlated with the amount of Stage 2 non-REM sleep they had gotten in the last quarter of the night.)