In this randomized , crossover , proof-of-concept study , single oral doses of the DORA almorexant ( 200 , 400 , and 1,000 mg ) were administered to healthy subjects with previous non-therapeutic experience with central nervous system depressants and were compared with placebo and single oral doses of zolpidem ( 20 and 40 mg ) , a benzodiazepine-like drug .