The present invention relates to compounds of formula wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are as defined herein andhetaryl is a one or two ring-membered heteroaromatic ring system, connected to the carbon atoms of the piperidine group selected from the group consisting of or 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.