Overnight verbal memory retention correlates with the number of sleep spindles.

Despite strong evidence supporting a role for sleep in the consolidation of newly acquired declarative memories, the contribution of specific sleep stages remains controversial. Based on electrophysiological studies in animals, synchronous sleep oscillations have been long proposed as possible origins of sleep-related memory improvement. Nevertheless, no studies to date have directly investigated the impact of sleep oscillations on overnight memory retention in humans. In the present study we provide evidence that overnight verbal memory retention is highly correlated with the number of sleep spindles detected by an automatic algorithm over left frontocentral areas. At the same time, overnight retention of newly learned faces was found to be independent of spindle activity but correlated with non-rapid-eye-movement sleep time. The data strongly support theories suggesting a link between sleep spindle activity and verbal memory consolidation.