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

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.