| Empirical work on the neural basis of mental activity in sleep has focused largely on describing and explaining differences between mentation reported from REM, as opposed to NREM sleep, an approach too often accompanied by the presumption that mechanisms specific to REM sleep constitute a complete model of the dreaming process. Yet such models have both failed to account for cognition outside of REM and to provide a detailed explanation of the mechanisms by which physiology in any sleep stage constructs the content of mentation.;In order to explore factors other than the REM/NREM cycle contributing to the production of mentation, we assessed the contribution of circadian and homeostatic factors to within-stage changes in sleep mentation across the diurnal cycle. Study 1 examines dreaming across the night in both REM and NREM, finding that time of night modulates the reporting of mentation at least as strongly as the ultradian sleep cycle, though some dream features change across the night selectively in REM. A follow-up study examines the characteristics of NREM mentation during daytime naps, in order to address the possibility that homeostatic-based changes in propensity for slow wave activity, rather than exclusively endogenously-driven circadian factors, contributed to the large time of night effect we report in Study 1. Overall, it is concluded that a purely circadian influence following the core body temperature cycle is inadequate to explain diurnal variations in mental activity during sleep. Two alternative neurobiological accounts are discussed.;The third study described here assesses the functioning of hippocampally-dependent declarative memory retrieval during NREM sleep and its role in generating sleep mentation. Evidence that neural "replay" of memories during NREM facilitates memory consolidation has naturally prompted speculation that NREM mentation may represent a cognitive component of sleep-dependent consolidation, but this assumption has thus far remained untested. Study 3 examines the expression of hippocampally-dependent "trace" conditioning during NREM and its relation to dream content. These findings confirm that recent declarative learning can be accessed during NREM, and furthermore suggest that hippocampally-mediated reactivation of declarative-type memory directly influences qualitative characteristics of ongoing sleep mentation. |
