Awakening protocol: all subjects were tested before lights out (PRE), REM and NREM awakening tests were performed across the night accompanied by a memory and word-stem task. Half of the subjects receive the tests 10 minutes into the second REM period (REMa) and forth NREM period (NREMa); half received the tests 10 minutes into the third NREM period (NREMb) and the third REM period (REMb). After awaking in the morning, all subjects were given the tasks again (POST) ((Reprinted from Cogn. Brain Res., 14, Walker, Liston, Hobson, and Stickgold, Cognitive flexibility across the sleep- wake cycle: REM-sleep enhancement of anagram problem solving, 317-324, Copyright (2002), with permission from Elsevier).