The present invention has been made based on the finding that a compound acting on the ORL-1 receptor as an agonist acts as a non-photic entrainment factor, and advances the circadian rhythm phase, and provides a novel therapeutic agent for a sleep disorder such as circadian rhythm sleep disorder, more particularly, an agent for the prophylaxis and/or treatment of a sleep disorder, which contains an ORL-1 receptor agonist, and a novel compound useful as such agent for the prophylaxis and/or treatment.