This invention relates to N-aroyl piperazine derivatives of formula (I), wherein: Y represents NR.sup.2; m represents 1, 2 or 3; p represents 0 or 1; X is O, S, C.dbd.O, SO.sub.2, or CH.dbd.CH--; Ar.sup.1 is aryl, or a mono or bicyclic heteroaryl group containing up to 4 heteroatoms selected from N, O and S; any of which may be optionally substituted; Ar.sup.2 represents phenyl or a 5- or 6-membered heterocyclyl group containing 1 to 3 heteroatoms selected from N, O and S, wherein the phenyl heterocyclyl group is substituted by R.sup.1 and further optional substituents; or Ar.sup.2 represents an optionally substituted bicyclic aromatic or bicyclic heteroaromatic group containing 1 to 4 heteroatoms selected from N, O and S; R.sup.1 represents optionally substituted (C.sub.1-4)alkoxy, halo, optionally substituted (C.sub.1-6)alkyl, optionally substituted phenyl, or an optionally substituted 5- or 6-membered heterocyclic ring containing 1 to 4 heteroatoms selected from N, O and S; R.sup.2 represents hydrogen, optionally substituted (C.sub.1-4)alkyl, optionally substituted (C.sub.1-6)alkanoyl, optionally subtituted (C.sub.1-6)alkanoxycarbonyl or optionally substituted aryl(C.sub.1-6)alkyloxycarbonyl; and their use as orexin receptor-antagonists.