Sleep and Human Aging

Abstract

Older adults do not sleep as well as younger adults. Why? What alterations in sleep quantity and quality occur as we age, and are there functional consequences? What are the underlying neural mechanisms that explain age-related sleep disruption? This review tackles these questions. First, we describe canonical changes in human sleep quantity and quality in cognitively normal older adults. Second, we explore the underlying neurobiological mechanisms that may account for these human sleep alterations. Third, we consider the functional consequences of age-related sleep disruption, focusing on memory impairment as an exemplar. We conclude with a discussion of a still-debated question: do older adults simply need less sleep, or rather, are they unable to generate the sleep that they still need?

Keywords: Alzheimer’s disease; aging; brain stimulation; dementia; memory; non-rapid eye movement sleep; rapid eye movement sleep; sleep; sleep spindles; slow waves.

Figure 1. Schematic of Age-Related Changes in Sleep Architecture and NREM Sleep Oscillations

(A) Prototypical sleep stage architecture across a 9 hr sleep period in a younger adult (top) and an older adult (bottom), using classic sleep staging criteria (Rechtschaffen and Kales, 1968). Relative to younger adults, older adults demonstrate: longer sleep latency, a greater number of transitions to lighter stages of sleep and wakefulness, more time spent awake after sleep onset, more fragmented sleep, and less time in slow wave sleep, especially within the early sleep cycles. (B) Upper: Representative topographical head plots of EEG-quantified differences between younger and older adults in slow wave activity (left upper) and density (right upper). A similar sleep spindle density for fast sleep spindles (13.5–15 Hz; bottom left) and slow sleep spindles (12–13.5 Hz; bottom right) is shown in the bottom image. The hotter colors represent higher values. The center rainbow topoplots in each image represent the subtracted difference between younger and older adults, with darker blue representing larger deficits in older relative to younger adults. For both slow waves and sleep spindles, older adults demonstrate the largest regional oscillation impairments over frontal EEG derivations. The data are adapted from previous reports (Mander et al., 2013, 2014, 2015, 2016b).

Figure 2. Age-Related Changes in Homeostatic Sleep Regulation

(A) In healthy, young adults, the increasing homeostatic drive to sleep (or “Process S”; green line in schematic graphic) is partially driven by increases in extracellular adenosine building up with accruing time awake (Porkka-Heiskanen et al., 1997). During sleep, adenosine, and consequential sleep pressure, decreases and does so in proportion to the intensity of slow wave activity during sleep (Borbély, 1982). In older adults (orange line), the homeostatic buildup of sleep pressure across a day (Process S) is slower and weaker in older adults relative to younger adults. Furthermore, the dissipation of homeostatic sleep drive is also shallower across a night of sleep in older relative to younger adults. (B) Left: Schematic of daytime subjective sleepiness, illustrating that older adults (orange) do not accumulate the same increase in subjective sleepiness across the day as younger adults (green). An illustration of the prototypical homeostatic discharge of SWA across sleep cycles at night is shown on the right. Younger adults express larger relative amounts of SWA in all sleep cycles, relative to older adults. Moreover, older adults show a shallower exponential decay of SWA across the night, relative to younger adults. (C) Schematic representation of the proposed age-related differences in extracellular adenosine signaling of sleep pressure (Process S) with accumulating time spent awake. Relative to younger adults, older adults demonstrated higher levels of adenosine accumulation with increasing time awake. However, due to a reduction in adenosine A1 receptors in aging, we hypothesize that an adenosine insensitivity develops. Therefore, despite higher levels of adenosine in older relative to younger adults, there is potentially a weaker adenosine-associated signaling in the aging brain, resulting in a lower sleep pressure signal in older relative to younger adults.

Figure 3. Gender Alters the Impact of Age on Sleep and Related Brain Structure and Function

(A) Representative changes in percent of the sleep period consisting of slow wave sleep in women (pink squares) and men (blue circles) across the lifespan (adapted from Ehlers and Kupfer, 1997; Redline et al., 2004). (B) Schematic of changes in the medial prefrontal cortex gray matter volume and resting metabolic rate in men and women (adapted from Kakimoto et al., 2016), a region known to underlie the source generation of NREM slow waves. Relative to younger adults (Y), older (O) adult men (blue) exhibit significantly more gray matter loss and reduced metabolic rate than women (pink), proposed as one candidate mechanism that contributes to these gender-dependent differences in slow wave sleep (described in A). (C) Changes in multiple hypothalamic nuclei in older adult (O) males (blue) and females (pink), relative to younger adult (Y) males (blue) and females (pink). Older adult males exhibit significantly greater relative loss of VIP neurons within the SCN and galanin-expressing neurons within the preoptic area of the hypothalamus. These cell changes offer one potential underlying cause of gender-specific changes in circadian and homeostatic sleep-wake regulation in older males than females.

Figure 4. Neuropathological Associations with Age-Related Impairments in Sleep Regulation and Sleep Architecture

(A) Schematic of sagittal view of brainstem and hypothalamic flip-flop model of sleep and wake regulating nuclei (Saper et al., 2010). The wake-promoting LHA (red) and LC (red) help to maintain stable periods of wake. The sleep promoting preoptic area (POA; blue) sends inhibitory projections to the LHA and the LC and, through this inhibition, helps to initiate and maintain stable sleep. The SCN (purple) further modulates this signal across a 24 hr period by fostering higher orexigenic activity in the LHA, promoting wakefulness during the day and decreases in influence permitting sleep at night. Due to age-related changes in these nuclei, the strength of sleep and wake promoting signals is compromised, resulting in an unstable brain state (lower right for older adults, relative to upper in younger adults). (B) Representation of age-related pathological differences in the nuclei that contribute to the flip-flop circuit of sleep-wake regulation. Relative to younger adults (green), older adults (orange) show reduced galanin-expressing preoptic area neurons, reduced VIP neurons in the SCN, reduced orexin-expressing neurons in the LHA, and reduced α-2 adrenoreceptor responsivity within the LC. These pathological changes may therefore contribute to greater sleep and wake instability in older adults, depicted in (A) bottom right. (C) Emerging from this proposed instability associated with these age-related changes within brainstem and hypothalamic sleep and wake regulatory centers is a collection of macro-level sleep alterations. These include decreased sleep duration, increased sleep latency, increased number of sleep stage transitions toward lighter sleep stages and wake, and greater fragmentation of consolidated periods of sleep and wake in older (orange) relative to younger (green) adults.

Figure 5. Structural Brain Changes and Age-Related Impairments in NREM Sleep Oscillations

Upper and middle: Age-related deficits in NREM slow wave amplitude and density are significantly associated with the degree of reduced gray matter volume and thickness (middle; blue regions) in the medial prefrontal cortex (mPFC), middle frontal gyrus (mFG), and insula in older (orange) relative to younger (green) adults (adapted from Dubé et al., 2015; Mander et al., 2013; Varga et al., 2016). Impairments in NREM sleep spindle duration and density (bottom right bar graph) reductions in older (orange) relative to younger (green) adults are associated with the magnitude of decreased hippocampal gray matter volume and white matter integrity within the body and splenium of the corpus callosum (middle; red regions; adapted from Fogel et al., 2016; Mander et al., 2016b) (lower image).

Figure 6. Hippocampus-Dependent Memory Decline Is Associated with Age-Related Sleep Disruption

(A) Sleep, including NREM fast sleep spindles, before learning supports the initial act of memory encoding (e.g., learning a face-name association) through a mediating influence on learning-dependent hippocampal activity. Age-related disruption of fast-frequency NREM sleep spindles over the prefrontal cortex attenuates this hippocampal encoding activity and thus the sleep-dependent learning restoration benefit (adapted from Mander et al., 2014). (B) Sleep, specifically NREM slow waves, after learning supports the subsequent offline consolidation of new memories, promoting hippocampal-neocortical memory transformation, resulting in hippocampal-independent retrieval the next day, minimizing overnight forgetting. Age-related impairment in NREM slow wave activity in the prefrontal cortex predicts worse overnight hippocampal-neocortical memory transformation, resulting in persistent, rather than progressively hippocampal-independent next-day retrieval activity. This is associated with accelerated forgetting and thus worse long-term memory retention (adapted from Mander et al., 2013).