Figure 1.

(A) Increase in sleep spindle density (mean ± SEM at Fz) during stage 2 sleep after a period of intensive declarative learning of paired-associate wordlists (filled circles) compared with an equally strenuous nonlearning condition (open circles). Differences were compared within subjects (^*P < 0.05; ^**P < 0.01). Large SEM values are attributable to individual differences, not differences between conditions. (B) During stage 2 sleep of the first cycle, nearly all subjects showed a substantial increase in spindle density after learning. Modified with permission from the Society for Neuroscience © 2002, Gais et al. 2002.