The present invention relates to compounds of formula I wherein Ar1, Ar2, Ar3, n, and R1 to R8 are as defined herein and to pharmaceutically suitable acid addition salts, optically pure enantiomers, racemates or diastereomeric mixtures thereof. These compounds are orexin receptor antagonists and may be useful in the treatment of disorders, in which orexin pathways are involved, like sleep disorders.