Abstract: The invention relates to 5,6,7,8-tetrahydro-imidazo[1,5-a]pyrazine derivatives of formula (I), wherein X represents CH2 or O; R1 represents a phenyl group, which group is independently mono-, di-, or tri-substituted wherein the substituents are independently selected from the group consisting of (C1-4)alkyl, (C1-4)alkoxy, halogen, cyano, trifluoromethoxy and trifluoromethyl; R2 represents (C1-4)alkyl, (C1-4)alkoxy, (C2-4)alkenyl, halogen, cyano, hydroxymethyl, trifluoromethyl, C(O)NR5R6 or cyclopropyl; R3 represents (C1-4)alkyl, (C1-4)alkoxy-methyl or halogen; R4 represents (C1-4)alkyl; R5 represents hydrogen or (C1-4)alkyl; and R6 represents hydrogen or (C1-4)alkyl. The invention also relates to pharmaceutically acceptable salts of such compounds; and to the use of such compounds as medicaments; especially as orexin receptor antagonists.