Relationship between sleep inertia and sleepiness: cumulative effects of four nights of sleep disruption/restriction on performance following abrupt nocturnal awakenings.

Performance deficits are usually evident following both extended wakefulness (sleep deprivation effects) and immediately upon awakening from sleep (sleep inertia effects). In order to determine whether sleep inertia effects are qualitatively different from sleep deprivation effects, performance on addition tests, Stanford Sleepiness Scale (SSS) ratings, and return-to-sleep latencies (RSLs) were assessed during four nights of sleep disruption/restriction. Eight subjects were polygraphically monitored in the sleep laboratory for five consecutive nights, from 2400 to 0700. On the last four nights (after an adaptation night) subjects were awakened at 0040, 0140, 0240, 0340, 0440, and 0540 for a 20-min test session. Sleepiness ratings and performance on 5-min addition tests were measured at 1.5, 7.5, and 13.5 min post-awakening, and RSL was measured at the end of each test session. Analysis of addition test performance across nights revealed that both speed and accuracy of calculations were adversely affected by the sleep disruption/restriction procedure, indicating that increasing sleepiness exacerbates sleep performance deficits upon awakening. Although divergence of SSS ratings and addition test performance across nights was suggestive, there was no conclusive evidence that sleep inertia is qualitatively different from "typical" sleepiness.