Study objectives: Functional significance of stage 2 sleep spindle activity for declarative memory consolidation.
Design: Randomized, within-subject, multicenter.
Setting: Weekly sleep laboratory visits, actigraphy, and sleep diary (4 weeks).
Participants: Twenty-four healthy subjects (12 men) aged between 20 and 30 years.
Interventions: Declarative memory task or nonlearning control task before sleep.
Measurement and results: This study measured spindle activity during stage 2 sleep following a (declarative) word-pair association task as compared to a control task. Participants performed a cued recall in the evening after learning (160 word pairs) as well as in the subsequent morning after 8 hours of undisturbed sleep with full polysomnography. Overnight change in the number of recalled words, but not absolute memory performance, correlated significantly with increased spindle activity during the experimental night (r24 = .63, P < .01). Time spent in each sleep stage could not account for this relationship.
Conclusion: A growing body of evidence supports the active role of sleep for information reprocessing. Whereas past research focused mainly on the distinct rapid eye movement and slow-wave sleep, these results indicate that increased sleep stage 2 spindle activity is related to an increase in recall performance and, thus, may reflect memory consolidation.