The memory sources of dreams: serial awakenings across sleep stages and time of night

Abstract

Memories of waking-life events are incorporated into dreams, but their incorporation is not uniform across a night of sleep. This study aimed to elucidate ways in which such memory sources vary by sleep stage and time of night. Twenty healthy participants (11 F; 24.1 ± 5.7 years) spent a night in the laboratory and were awakened for dream collection approximately 12 times spread across early, middle, and late periods of sleep, while covering all stages of sleep (N1, N2, N3, REM). In the morning, participants identified and dated associated memories of waking-life events for each dream report, when possible. The incorporation of recent memory sources in dreams was more frequent in N1 and REM than in other sleep stages. The incorporation of distant memories from over a week ago, semantic memories not traceable to a single event, and anticipated future events remained stable throughout sleep. In contrast, the relative proportions of recent versus distant memory sources changed across the night, independently of sleep stage, with late-night dreams in all stages having relatively less recent and more remote memory sources than dreams earlier in the night. Qualitatively, dreams tended to repeat similar themes across the night and in different sleep stages. The present findings clarify the temporal course of memory incorporations in dreams, highlighting a specific connection between time of night and the temporal remoteness of memories. We discuss how dream content may, at least in part, reflect the mechanisms of sleep-dependent memory consolidation.

Keywords: NREM sleep; REM sleep; dreaming; episodic memory; memory sources; semantic memory; serial awakenings; sleep stages; temporal changes.

Graphical Abstract

Figure 1.

Example of a participant’s hypnogram with serial awakenings; each awakening is indicated with an arrow. Wake periods are in black and thirds of the night are indicated with horizontal brackets.

Figure 2.

Dream recall variation across Sleep stage and Time of night. (A) Dreams were more frequently recalled in REM sleep across the night; (B) dreams were generally longer in REM sleep, especially in later awakenings; (C) dreams were remembered more clearly in the morning when they occurred in REM or in N1 sleep, or when they occurred in later awakenings. Means and SEMs are shown.

Figure 3.

Memory sources of dreams. (A) Examples of episodic, semantic, and future sources associated with dream reports. (B) Total number of memory sources identified within a dream, including episodic, semantic, and future sources (REM>N2, N3; N1>N3). Boxplots with Min to Max point ranges are shown. (C) Percentage of dreams, by Sleep stage, that were associated with memory sources from specific time periods or of specific type (semantic, future). Recent episodic memory sources, and day residues specifically, were more likely to be identified in N1 and REM dreams compared to other sleep stages.

Figure 4.

Proportion of memory sources in dreams. (A) Proportions in dreams of episodic past-distant, episodic past-recent, semantic and future memory sources across Time of night. (B) Relative difference between the proportions of recent-past and distant-past episodic memory sources by Time of night and (C) by both Sleep stages and Time of night. Means and SEMs are shown in A and C. Boxplots with Min to Max point ranges are shown in B. *p < .05.

Figure 5.

Examples of repeated themes in dreams associated with individual memory sources throughout the night. The dots represent all awakenings performed during the night for dream collection in these participants (black dots = dream recall; empty dots = no dream recall; the positions of the dots are approximate and for illustrative purpose only). Dreams originally associated with the memory sources displayed on the left are marked with (*). Some participants had more than one memory source associated with the repeated theme; only one or two memory sources by theme is shown here.