The present invention provides a therapeutic and/or preventive agent for a sleep disorder comprising, as an active ingredient, a thiazole derivative represented by the general formula (I), [wherein R 1 represents a five-membered aromatic heterocyclic group including at least one oxygen atom, or the like, R 2 represents halogen or the like, and R 3 represents -NR 10 R 11 (wherein R 10 and R 11 may be the same or different, and each represents a hydrogen atom or the like), or the like] or a pharmaceutically acceptable salt thereof.
