Abstract
In this thesis, we investigated the role of sleep in the consolidation of declarative memory in humans and addressed several issues that have not been studied in detail in the existing literature, including (a) whether declarative memory consolidation can be viewed as involving two processes, namely memory enhancement and stabilization, (b) whether the semantic relatedness, the level of learning, and the amount of declarative materials acquired modifies subsequent offline memory processing during sleep, (c) whether the consolidation of declarative and procedural memory during sleep is supported by the same or different systems, (d) whether having a nap immediately after learning facilitates declarative memory consolidation to the same extent as a delayed nap, and (e) whether sleep duration and architecture is related to offline declarative memory processing. This thesis consists of five studies, and we examined the effects of nocturnal sleep in Study 1 (N = 60) and Study 2 (N = 20), while the effects of daytime napping were investigated in Study 3 (N = 20), Study 4 (N = 20), and Study 5 (N = 32). Our results indicated that declarative memory consolidation consists of both enhancement and stabilization. Furthermore, sleep plays an important role in the enhancement of poorly acquired, hippocampally independent declarative associations as well as the stabilization of moderately acquired, hippocampally dependent declarative associations, but this facilitative role of sleep in the offline processing of declarative memory diminishes as the declarative materials become better learned. We also found that the enhancement and stabilization of declarative memory during sleep requires brain mechanisms and resources that differ from those involved in the consolidation of procedural memory, is independent of the amount of learning materials acquired at each level of learning, cannot be accounted for by the duration of the post-learning sleep episode or any particular sleep stage, and is independent of the timing of the sleep episode relative to the end of learning.