Comparing sleep-loss sleepiness and sleep inertia: lapses make the difference

To compare the behavioral effects of sleep-loss sleepiness (performance impairment due to sleep loss) and sleep inertia (period of impaired performance that follows awakening), mean response latencies and number of lapses from a visual simple reaction-time task were analyzed. Three experimental conditions were designed to manipulate sleepiness and sleep-inertia levels: uninterrupted sleep, partial sleep reduction, and total sleep deprivation. Each condition included two consecutive nights (the first always a night of uninterrupted sleep, and the second either a night of uninterrupted sleep, a night when sleep was reduced to 3 h, or a night of total sleep deprivation), as well as two days in which performance was assessed at 10 different time points (08:00, 08:30, 09:00, 09:30, 10:00, 11:00, 14:00, 17:00, 20:00, and 23:00 h). From 08:00 to 09:00 h, reaction times in the partial sleep-reduction and total sleep-deprivation conditions were at a similar level and were slower than those observed in the uninterrupted sleep condition. In the same time period, the frequency of lapses in the total sleep-deprivation condition was higher than in the partial sleep-reduction condition, while this latter condition never differed from the uninterrupted sleep condition. The results indicate that both sleep inertia and sleep-loss sleepiness lead to an increase in response latencies, but only extreme sleepiness leads to an increase in lapse frequency. We conclude that while reaction times slow as a result of both sleep inertia and sleep-loss sleepiness, lapses appear to be a specific feature of sleep-loss sleepiness.