Abstract
Numerous studies have examined sleep's influence on a range of hippocampus-dependent declarative memory tasks, from text learning to spatial navigation. In this study, we examined the impact of sleep, wake, and time-of-day influences on the processing of declarative information with strong semantic links (semantically related word pairs) and information requiring the formation of novel associations (unrelated word pairs). Participants encoded a set of related or unrelated word pairs at either 9am or 9pm, and were then tested after an interval of 30 min, 12 hr, or 24 hr. The time of day at which subjects were trained had no effect on training performance or initial memory of either word pair type. At 12 hr retest, memory overall was superior following a night of sleep compared to a day of wakefulness. However, this performance difference was a result of a pronounced deterioration in memory for unrelated word pairs across wake; there was no sleep-wake difference for related word pairs. At 24 hr retest, with all subjects having received both a full night of sleep and a full day of wakefulness, we found that memory was superior when sleep occurred shortly after learning rather than following a full day of wakefulness. Lastly, we present evidence that the rate of deterioration across wakefulness was significantly diminished when a night of sleep preceded the wake period compared to when no sleep preceded wake, suggesting that sleep served to stabilize the memories against the deleterious effects of subsequent wakefulness. Overall, our results demonstrate that 1) the impact of 12 hr of waking interference on memory retention is strongly determined by word-pair type, 2) sleep is most beneficial to memory 24 hr later if it occurs shortly after learning, and 3) sleep does in fact stabilize declarative memories, diminishing the negative impact of subsequent wakefulness.
Highlights
The discovery that sleep benefits the retention of human memories dates back to Ebbinghaus [1], who inaugurated contemporary memory research with his seminal study of forgetting
Jenkins and Dallenbach [2] experimentally confirmed that memory retention following sleep was superior to retention following an equivalent period of wakefulness
In this study we examined the impact of sleep, wake, and the time-of-day on memory for declarative information with strong semantic links and information requiring the formation of novel associations
Summary
The discovery that sleep benefits the retention of human memories dates back to Ebbinghaus [1], who inaugurated contemporary memory research with his seminal study of forgetting. Protection from interference likely plays a role in the sleep effect, too few studies have employed research designs capable of parsing the unique contributions of sleep and wakefulness to memory processing, with many studies examining performance after a single 12 hr training-retest interval containing just one night of sleep vs one day of wakefulness. Benson & Feinberg (1975, 1977), using 8, 16, and 24 hr trainingretest intervals, found that participants who slept soon after learning a list of unrelated paired associates retained more information 24 hours later than participants who endured a full day of waking activities prior to sleeping, while participants trained instead on nonsense syllables showed no difference between conditions [7,8] This delayed testing procedure was used to study learning of English-German vocabulary lists in high school students [9]. An additional benefit of testing both types of memory is that it permits us to draw broader and stronger conclusions about the roles of sleep and wake on hippocampus-dependent memory consolidation
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