Time-course of sleep inertia upon awakening from nighttime sleep with different sleep homeostasis conditions

Background: We assessed the time-course of sleep inertia during the first 7 5 min after morning awakening from regular nocturnal sleep, as well as from nighttime sleep episodes with altered sleep homeostasis conditions. Method s: Ten normal males slept for 6 nights in the laboratory: 1 adaptation (AD) , 2 baseline (BSL, BSL-A), 2 selective Slow-Wave Sleep (SWS) deprivation (D EP-1, DEP-2), and 1 recovery night (REC). On morning awakening, performance was assessed by means of: a) Descending Subtraction Task (DST); b) Auditor y Reaction Time task (ART); and c) Finger Tapping Task (FTT). The test batt ery, lasting about 13 min, was repealed for 5 times. Results: In regard to DST, the Correct Response ratio (CR/NR) showed a great increase of sleep in ertia on the first testing session of REC. Regarding sleep inertia time-cou rse, a significant linear decrease across the testing sessions during the B SL-A and the DEP-2 was present, whereas a significant quadratic trend durin g the AD, the DEP-1 and the REC was found. On the other hand, ART performan ce showed a significant quadratic trend across testing sessions, while FTT performance did not show any significant variation. Conclusions: A uniform pattern of variation of time-course of sleep inertia as a function of the d ifferent sleep homeostasis conditions was nor recognized. Performance accur acy (CR/NR) on the DST showed the hypothesized increasing linear trend acro ss testing sessions only during 2 out of b nights, while the unexpected qua dratic trend of ART performance is probably due to a fatigue effect. During sleep inertia, cognitive performance reached the baseline level about 30 m in after awakening, while motor performance was still below the baseline le vels 75 min after awakening. The finding that cognitive performance recover y is greater and more rapid than motor performance recovery could be very i mportant for operational settings and in sustained operations.