['Modafinil may promote wakefulness by increasing cerebral dopaminergic neurotransmission, which importantly depends on activity of catechol-O-methyltransferase (COMT) in prefrontal cortex. The effects of modafinil on sleep homeostasis in humans are unknown. Employing a novel sleep-pharmacogenetic approach, we investigated the interaction of modafinil with sleep deprivation to study dopaminergic mechanisms of sleep homeostasis.', 'Placebo-controlled, double-blind, randomized crossover study.', 'Sleep laboratory in temporal isolation unit.', '22 healthy young men (23.4 +/- 0.5 years) prospectively enrolled based on genotype of the functional Val158Met polymorphism of COMT(10 Val/Val and 12 Met/Met homozygotes).', '2 x 100 mg modafinil and placebo administered at 11 and 23 hours during 40 hours prolonged wakefulness.', 'Subjective sleepiness and EEG markers of sleep homeostasis in wakefulness and sleep were equally affected by sleep deprivation in Val/Val and Met/Met allele carriers (placebo condition). Modafinil attenuated the evolution of sleepiness and EEG 5-8 Hz activity during sleep deprivation in both genotypes. In contrast to caffeine, modafinil did not reduce EEG slow wave activity (0.75-4.5 Hz) in recovery sleep, yet specifically increased 3.0-6.75 Hz and > 16.75 Hz activity in NREM sleep in the Val/Val genotype of COMT.'] Do effects of modafinil on the sleep EEG depend on Val158Met genotype of COMT?