Differences in Sleep Quality and Timing Influence Long Term Forgetting of Verbal Episodic Memory, and Face Processing

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We examined cognition-specific effects of sleep quality and timing in 145 cognitively-normal subjects (M=55.3 years), using self-report (Pittsburgh Sleep Quality Index, PSQI) and actigraphy, testing whether NREM-rich sleep supports long-term consolidation of declarative memory, while REM-rich sleep supports facial processing. Good sleep is thought to improve declarative memory consolidation, but its potential role in longer-timescale forgetting (e.g., 2 weeks after the learning event) is largely unexplored. We modified a standard test of verbal episodic memory (VEM) to test 2-week retention relative to 30-minute delay performance. Subjects also completed other cognitive tasks and psychological questionnaires. We analysed predictors of VEM Long-Term Forgetting and Facial processing. Good quality sleep (PSQI), good psychological health (questionnaires) and being an early sleeper (actigraphy), i.e. likely obtaining more NREM-rich sleep, predicted better/lower VEM Long-Term Forgetting. Conversely, being a late sleeper (actigraphy), i.e. likely obtaining more REM-rich sleep, predicted better Facial processing. To our knowledge, this is the first indication that sleep quality influences long-timescale (2-week) consolidation of a single learning episode, and that later-sleeping community-living subjects have superior facial processing. These novel findings are consistent with research showing associations between good sleep quality and higher NREM sleep, and between early-stage NREM sleep and long-term memory consolidation. Since the benefits of good sleep for long-term forgetting applied to older as well as younger participants, these findings also re-emphasise the importance of sleep health in older adults.