Introduction Good sleep is thought to facilitate memory consolidation, yet its potential role in long-term memory (e.g., 2 weeks after the learning event) is largely unexplored. We examined if good sleep would predict lower long-term forgetting by modifying a standard neuropsychological test of verbal episodic memory (VEM) to include re-test without notice after two weeks.
Methods 145 cognitively-normal subjects (M=55.3 years, SD=17.1 years) undertook two separate phases of memory-domain testing two weeks apart. Subjects completed the Pittsburgh Sleep Questionnaire (PSQ), and also wore research-grade accelerometers which measured physical activity and sleep in the two-week period between testing phases. Subjects were divided between younger (N=52, M=36 years) and older (N=93, M=66 years) and also between PSQ-determined good and poor sleepers.
Results Factor analysis organised 12 measures from 7 memory tests into four memory-domain factors. Good self-reported sleep predicted better performance in the memory-domain factor for VEM long-term forgetting, but not in the other three memory factors (standard neuropsychological measures of VEM short-term recall, face memory and working memory), which were instead predicted (negatively) by age. The double dissociation between VEM Short-term recall and VEM long-term forgetting is illustrated in the figure 1. Accelerometry-based sleep measures were not associated with memory. Exploratory analysis of psychological health scores (see table 1) showed that better scores in a factor derived from the SF36 Short Form Health Survey also helped to predict lower long-term forgetting.
Discussion Good PSQ-rated sleep quality and better psychological health may both help to improve performance in a new cognitive measure of long-term forgetting in younger and older persons. The finding suggests that PSQ-rated sleep quality may be a biomarker for long-term memory consolidation processes. To our knowledge, this is the first indication that sleep quality may determine long-term (2-week) consolidation of a single learning episode.
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