Sleep inertia is a process from sleep to fully wake up. The transition entails a temporary period of confusion, disorientation, reduced alertness, bad affect and poor task performance. Previous studies have focused more attention on the pharmacological ways (e.g., caffeine, excitant) of counteracting sleep inertia. However, there is little study investigating the non-pharmacological ways such as light exposure on the impact of sleep inertia post awakening up to now. The aim of the current study was using behavior and PSG technique to explore the effect of morning sleep inertia on subjective alertness and mood as well as cognitive performance in young healthy adults. We also examined whether exposure to dynamic dawn-simulated light could be a valid countermeasure against the detrimental effects of sleep inertia after waking up in the morning. A single factor within-subject design with lighting condition (at eye level) as independent variable was used in the present study. Three different light settings (separated by 1wks) were administered each morning after one 6-h sleep restriction night: a dawn-simulating light (starting 30min before scheduled wake-up time, polychromatic light gradually increasing from 0 to 210lux), a constant light (40lux for 30min before scheduled wake-up time) and a control condition (no any light exposure prior to wake up). 21 college students (9 males) participated in the study based on strict selection criteria such as chronotype, medical and sleep disorders, general health, life habits, affective disorders, smoking, drug consumption, body mass index, shift work and transmeridian ?ights during last two months. Sleep inertia was measured by a 2-h test protocol since wakeup, including subjective ratings of alertness and mood, and by performance on a psychomotor vigilance task (PVT), a 2-back and a go/no-go task. Results were as follows: A paired t-test between baseline and the first post-sleep assessment revealed a significant difference in both KSS and PANAS scores such that subjective alertness and positive mood immediately after awakening were much lower than at baseline, thereafter it declined over a two-hour period as indicated by a significant effect of time. Performance on PVT, 2-back and go/no-go tasks were founded to be significantly worse at the first test after waking up than at baseline, and the effect of sleep inertia on these tasks dissipated over the course of two hours as indicated by a significant effect of time in the analysis of variance. Moreover, performance speed was more impaired than accuracy. Compared with control and constant light condition, the dawn simulation light resulted in a significant improvement of subjective alertness and positive mood after waking up. Similarly, response times on PVT and accuracy on go/no-go task were much faster and higher respectively in the artificial dawn condition relative to the control and constant light. However, the light conditions had no differential effects on negative mood and the 2-back task. These findings suggest that morning sleep inertia had a significant negative effect on subjective alertness and positive affect as well as some cognitive functions like sustained attention, working memory and inhibitory capacity. The use of simulated dawn light could to some extent help counteract sleep inertia after awakening in the morning, which effect was highly related to the light pattern with a dynamic exponential change.