Aging in mice reduces the ability to sustain sleep/wake states

Abstract

One of the most significant problems facing older individuals is difficulty staying asleep at night and awake during the day. Understanding the mechanisms by which the regulation of sleep/wake goes awry with age is a critical step in identifying novel therapeutic strategies to improve quality of life for the elderly. We measured wake, non-rapid eye movement (NREM) and rapid-eye movement (REM) sleep in young (2-4 months-old) and aged (22-24 months-old) C57BL6/NIA mice. We used both conventional measures (i.e., bout number and bout duration) and an innovative spike-and-slab statistical approach to characterize age-related fragmentation of sleep/wake. The short (spike) and long (slab) components of the spike-and-slab mixture model capture the distribution of bouts for each behavioral state in mice. Using this novel analytical approach, we found that aged animals are less able to sustain long episodes of wakefulness or NREM sleep. Additionally, spectral analysis of EEG recordings revealed that aging slows theta peak frequency, a correlate of arousal. These combined analyses provide a window into the mechanisms underlying the destabilization of long periods of sleep and wake and reduced vigilance that develop with aging.

Figure 1. Aged mice show reduced wakefulness during dark phase and decreased REM sleep during light phase.

A. Total time (min) spent awake for young and old mice over 24 hours (left) and during the light and dark period (right). B. Percent time spent awake shown in 8×3-hour bins across light/dark cycle. Solid line represents the dark period C. Aged mice show increased NREM sleep during the dark phase. Total time spent in NREM sleep over 24 hours (left) and during the light and dark period (right). D. Percent time spent in NREM in 3-hour bins. E. Old mice have decreased REM sleep during the rest period. Total time spent in REM sleep for young and old mice over 24 hours (left) and during the light and dark period. F. Percent time spent in REM sleep shown in 3-hour bins across light/dark cycle. Mean ± standard error of the mean (s.e.m.), * p<0.05.

Figure 2. Aging causes fragmentation of wake and NREM sleep.

A. Number of bouts of wakefulness (left) and average wake bout duration (right) during the light and dark phase. Aged mice show more bouts of wakefulness of shorter average duration during the dark phase. B. Aged mice show fragmented NREM sleep during the dark phase. Number of NREM bouts (left) and average NREM bout duration (right) during the light and dark phase. C. Number of REM episodes (left) and average REM bout duration during the light and dark period. Mean ± s.e.m., * p<0.05.

Figure 3. Spike and slab model shows improved fit over a single distribution.

Distribution of NREM to Wake bout durations are shown for young and old mice during the light (A) and dark period (B). The solid line represents the density of the spike and slab model, and the dashed line represents the density of the traditional gamma model. The gamma model assigns too little probability to short bouts and too much probability to long bouts.

Figure 4. Aged mice are unable to sustain long bouts of wakefulness and NREM sleep.

A. Number of bouts during the dark phase for young and old mice transitioning from NREM to wake (left) and from wake to NREM (right). Aged mice show an increased number of episodes for both states. B. Average duration of slabs (long bouts) for mice transitioning from NREM to wake (left) and wake to NREM (right). The duration of the slabs for both transitions is reduced in aged animals. Mean ± s.e.m., ‡ p<0.00625 (Bonferroni correction).

Figure 5. Aging slows theta peak frequency (TPF) and decreases slow wave activity (SWA) at low frequencies (0.5–1.5 Hz).

A. Wake EEG spectra calculated during the dark period for young and old mice. Aged animals show lower power in the higher frequency range. B. EEG spectra for NREM sleep generated during the light period for young and aged mice. Aging did not profoundly affect NREM spectral profile. C. EEG spectra of REM sleep computed during the light phase for young and old mice. D. Theta peak frequency (TPF) for wake and REM EEG during the dark and light phase, respectively. TPF slowed with aging for both states. E. Slow wave activity (0.5–4 Hz, SWA) of NREM sleep for young and old mice expressed relative to the last 4 hours of the light period ( = 100%) for each animal. Grey area denotes the dark period. F. Low frequency (0.5–1.5) SWA of NREM sleep young and old mice expressed relative to the last 4 hours of the light period ( = 100%) for each animal. Grey area denotes the dark period. Mean ± s.e.m., gray bar and * indicate p<0.05.