Sleep, dreams, and memory consolidation: The role of the stress hormone cortisol

Abstract

We discuss the relationship between sleep, dreams, and memory, proposing that the content of dreams reflects aspects of memory consolidation taking place during the different stages of sleep. Although we acknowledge the likely involvement of various neuromodulators in these phenomena, we focus on the hormone cortisol, which is known to exert influence on many of the brain systems involved in memory. The concentration of cortisol escalates over the course of the night's sleep, in ways that we propose can help explain the changing nature of dreams across the sleep cycle.There is currently no convincing explanation for why we dream or what we dream about. In this article we propose an approach to dreaming that focuses on the relationship between sleep and memory. 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(Winson , 2002(Winson , 2004Kali and Dayan 2004).It is generally assumed that long-term memory consolidation involves interactions among multiple brain systems, modulated by various neurotransmitters and neurohormones. 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. 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.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. The status of these circuits largely determines the phenomenology of dreams, providing an explanation for why we dream and of what. 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.In addition to exploring these ideas in more detail, we provide some background concerning (1) the states of sleep and the role of various neurotransmitters in switching from one sleep state to another, (2) how the characteristics of dreams vary as a function of sleep state, (3) the memory content typically associated with dreaming in different dream states, and (4) the role of sleep in the consolidation of memory.

Association of salivary-assessed oxytocin and cortisol levels with time of night and sleep stage

“…This study is relevant to proposals for mnemonic consolidation functions of REM sleep and of N2, in that it indicates that during the later part of sleep CT is as high in N2 as it is in REM. If REM memory-consolidation processes late in sleep are subject to increased CT levels at that time, as proposed by Payne and Nadel (2004), Wagner and Born (2008) and Payne (2010), then N2 memory-consolidation mechanisms (Peters et al 2008;Fogel and Smith 2010) may be subject to the same cortisol-induced processes. Cortisol can have a disruptive effect on learning and memory (e.g.…”

“…However, study of the differential contribution of these stages of sleep to the various types of memory is confounded by the changing levels of CT in the night, as CT increases at the time of night that REM sleep becomes predominant. Furthermore, CT not only can impede memory consolidation and retrieval (Payne and Nadel 2004;Wagner 2004, 2007;Payne 2010) but can also enhance encoding and consolidation of emotionally salient information (de Quervain et al 2009). This study is relevant to proposals for mnemonic consolidation functions of REM sleep and of N2, in that it indicates that during the later part of sleep CT is as high in N2 as it is in REM.…”

“…There have been proposals that the increased cortisol in the second part of the night is a functional component of emotional memory consolidation during REM sleep (Payne and Nadel 2004;Wagner and Born 2008;Payne 2010). We thus aimed to compare cortisol (CT) levels following awakenings from REM sleep with those following N2 sleep, hypothesising that REM sleep CT would be higher than N2 CT. A second proposal for a function of REM sleep is that it is involved in promoting attachment behaviour (McNamara et al 2001).…”

Elevated Sleep Spindle Density after Learning or after Retrieval in Rats

“…Sleep spindles are generated in corticothalamic circuits, which are extensively involved in sensory processing. Their temporal correlation with hippocampal ripples (Siapas and Wilson, 1998) provide a link between hippocampal "episode" traces and neocortical "feature" traces (Pennartz et al, 2002;Kali and Dayan, 2004;Payne and Nadel, 2004). The off-line replay of learning-associated neuronal patterns that are temporally coordinated by brain rhythms can promote the corticothalamic-hippocampal dialogue necessary for memory consolidation (Buzsaki, 1996;Sirota et al, 2003;Moelle et al, 2006;Steriade, 2006).…”

Evidence for the preferential incorporation of emotional waking-life experiences into dreams.

“…Similarly, Desseilles et al (2010) postulated that the novel and/or "individually-relevant" (p.1000) features of waking-life experiences may be selectively incorporated for consolidation and integration into the memory system. Likewise, other researchers have hypothesised that dreams may reflect the 7 consolidation of novel information: for example, Payne and Nadel (2004) suggested that dreams reflect the memory integration processes of weaving in new information into old material, and Stickgold et al (2001) suggested that dreams reflect the process of novel cortical associations being identified and evaluated in the brain. These ideas all indicate that novel and/or personally significant experiences may be preferentially incorporated into dreams in order to be assimilated into and/or evaluated by the wider memory system.…”