As a corollary, dreams can be thought of as windows onto the inner workings of our memory systems, at least those of which we can become conscious. The status of these circuits largely determines the phenomenology of dreams, providing an explanation for why we dream and of what. Our hypothesis, briefly stated, is that variations in cortisol (and other neurotransmitters) determine the functional status of hippocampal ↔ neocortical circuits, thereby influencing the memory consolidation processes that transpire during sleep. Although a number of neurotransmitters and neurohormones are likely involved, we focus our attention in particular on the stress hormone cortisol, which has widespread effects on memory during waking life through its impact on many of the critical brain structures implicated in memory function.
We propose that the characteristics of dreams are best understood in the context of this neuromodulatory impact on the brain systems involved in memory consolidation. It is generally assumed that long-term memory consolidation involves interactions among multiple brain systems, modulated by various neurotransmitters and neurohormones. We suggest that dreams reflect a biological process of long-term memory consolidation, serving to strengthen the neural traces of recent events, to integrate these new traces with older memories and previously stored knowledge, and to maintain the stability of existing memory representations in the face of subsequent experience (Winson 1985, 2002, 2004 Kali and Dayan 2004). In this article we propose an approach to dreaming that focuses on the relationship between sleep and memory. There is currently no convincing explanation for why we dream or what we dream about.
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