A compound represented by the formula (I) or a salt thereof; and an agent for protecting a brain/neuronal cell or a therapeutic agent for sleep disorder comprising the compound or salt: wherein Z represents an oxygen or sulfur; R1 represents an aryl or heterocyclic group which may be substituted, provided that R1 is not a 3-ethyl-6-methoxy-2-methyl-5-naphthalen-1-yl group; R1a represents a hydrogen atom, a hydrocarbon group which may be substituted, a hydroxyl group or the like; R2 represents a piperidin-1,4-diyl or piperazin-1,4-diyl which may be substituted; R3 represents a bivalent group which is formed by removing two hydrogen atoms from a benzene ring or 6-membered aromatic heterocyclic ring which may have a substituent, provided that R3 is not a pyridazin-3,6-diyl; and R4 represents a group which is formed by removing one hydrogen atom from a benzene ring or 5- to 6-membered heterocyclic ring which may be substituted, provided that the substituent on the heterocyclic ring is not a phenylethyl group which may be substituted when Z is sulfur.