Sleep inertia varies with circadian phase and sleep stage in older adults

Abstract

The purpose of our analysis was to determine if older adults show sleep inertia effects on performance at scheduled wake time, and whether these effects depend on circadian phase or sleep stage at awakening. Using the Digit Symbol Substitution Test, effects of sleep inertia on performance were assessed over the first 30 min after wake time on baseline days and when sleep was scheduled at different circadian phases. Mixed model analyses revealed that performance improved as time awake increased; that beginning levels of performance were poorest when wake time was scheduled to occur during the biological night; and that effects of sleep inertia on performance during the biological night were greater when awaking from non-REM (NREM) sleep than from REM sleep. Based on our current understanding of sleep inertia effects in young subjects, and previous reports that older subjects awaken at an earlier circadian phase and are more likely to have their final awakening from NREM sleep than younger adults, our findings suggest older adults may be more vulnerable to sleep inertia effects than young adults.

Figures

Figure 1.

Scheduled sleep-wake cycle of the study protocol plotted in double raster format, with successive days plotted to the right of, and beneath one another. Reference clock hour is indicated along the top axis. Scheduled sleep episodes are represented by the black bars. The first three baseline sleep episodes were scheduled to occur at each subject’s habitual bedtime and last for 8 hours. Beginning on the fourth sleep episode, subjects began the forced-desynchrony (FD) segment of the study, during which they were scheduled to sleep for 6.67 hours, with sleep scheduled to occur 4 hours earlier each day. The arrows mark the first (on Day 5) and last (on Day 18) days of FD sleep inertia testing.

Figure 2.

Performance across the first 30 minutes of scheduled wake during baseline (open symbols) and forced-desynchrony (filled symbols) conditions. The number of DSST trials completed correctly (mean ± sem) for each of the four test batteries (scheduled to occur 1, 10, 20 and 30 minutes after wake time) are plotted with respect to the time at which each test battery actually occurred.

Figure 3.

Performance across the first 30 minutes of scheduled wake time during forced-desynchrony plotted with respect to circadian phase at scheduled wake time. Scheduled wake times were binned into 60° circadian phase bins (approximately 4 circadian hours), with 0° corresponding to the core body temperature minimum. Within each circadian phase bin, the number of correct DSST trials (mean + sem) from each of the four sleep inertia batteries are shown connected with a solid line. Note that within each 4-hour circadian phase bin, the 30 minutes of performance data for that bin are presented on an expanded scale, so that the change in performance across the initial 30 minutes of awakening can be better visualized. Data are double plotted, and the initial bin at each circadian phase is shown connected with a dashed line to better visualize the circadian variation of performance upon awakening.

Figure 4.

Performance across the first 30 minutes of scheduled wake time during forced-desynchrony upon awakening from REM (top panel) or NREM Stages 1 or 2 sleep (bottom panel). As in Figure 3, scheduled wake times were binned into 60° circadian phase bins, with 0° corresponding to the core body temperature minimum. Within each circadian phase bin, the number of correct DSST trials (mean + sem) from each of the four sleep inertia batteries are shown connected with a solid line. Data are double plotted, and the initial bin at each circadian phase is shown connected with a dashed line to better visualize the circadian variation of performance immediately upon awakenings. There were no wake times from REM sleep at 180° or 240° which met our criteria for inclusion in the analysis.