How do the sleep features that characterise depression impact memory?

Abstract

Depression is associated with general sleep disturbance and abnormalities in sleep physiology. For example, compared with control subjects, depressed patients exhibit lower sleep efficiency, longer rapid eye movement (REM) sleep duration, and diminished slow-wave activity during non-REM sleep. A separate literature indicates that depression is also associated with many distinguishing memory characteristics, including emotional memory bias, overgeneral autobiographical memory, and impaired memory suppression. The sleep and memory features that hallmark depression may both contribute to the onset and maintenance of the disorder. Despite our rapidly growing understanding of the intimate relationship between sleep and memory, our comprehension of how sleep and memory interact in the aetiology of depression remains poor. In this narrative review, we consider how the sleep signatures of depression could contribute to the accompanying memory characteristics.

Keywords: REM sleep dysregulation; categorical memory; cognitive bias; major depressive disorder (MDD); memory control.

Figure 1.. Depiction of typical sleep and sleep in depression.

(A) Sleep can be broadly classified into two main types: rapid eye movement (REM) sleep, and non-rapid eye movement (NREM) sleep. The latter can be further divided into three stages, with each stage signifying progressively deeper levels of sleep. The deepest stage of NREM sleep is often referred to as slow-wave sleep (SWS) because it is defined by the presence of low-frequency neural oscillations (<4 Hz). In contrast, REM sleep is defined by the presence of darting eye movements and is associated with comparatively fast, mixed-frequency oscillations (∼4–25 Hz). Across the night, NREM and REM sleep occur sequentially in ∼90 min cycles. The ratio of SWS to REM sleep shifts throughout the night, with most SWS occurring in the first half of the night, and most REM sleep occurring in the second half of the night. (B) The overall structure of sleep is broadly similar in depressed patients. However, compared with non-depressed individuals, depressed patients exhibit (i) shorter REM sleep latency (i.e. they enter REM sleep more quickly following sleep onset), (ii) longer REM sleep duration (i.e. they spend more time in REM sleep), and (iii) shorter SWS duration (i.e. they spend less time in SWS). Notably, depressed patients also exhibit greater REM density (i.e. more rapid eye movements during REM sleep) and exhibit less low-frequency (<4 Hz) brain activity during SWS (not depicted). Abbreviations: N1 & N2, light stages of NREM sleep.

Figure 2.. An overview of possible links between the sleep and memory features that characterise depression.

Solid arrows reflect links that are reasonably well-established by experimental data, whereas dotted arrows reflect links that are indirect or less established. Additional links are possible and may emerge through further empirical investigation. Moreover, bi-directional links are likely (e.g. deficient memory suppression could lead to general sleep disturbance), but beyond the scope of the present article. Abbreviations: REM, rapid eye movement; SWS, slow-wave sleep; N1 & N2, light stages of NREM sleep; OGM, overgeneral autobiographical memory.