Replacement of the dimethoxyphenyl moiety in the core skeleton of almorexant by appropriately substituted imidazoles afforded novel 1-chloro-5, 6, 7, 8-tetrahydroimidazo [1, 5-a] pyrazine derivatives as potent dual orexin receptor antagonists. We describe in this Letter our efforts to further optimize the potency and brain penetration of these derivatives by fine-tuning of the pivotal phenethyl motif, and we comment on the sleep-promoting activity ...