The invention relates to a once a day formulation of trazodone or a trazodone derivative. The formulation contains trazodone or a trazodone derivative and a controlled release excipient so that, once administered orally, the trazodone or the trazodone derivative is maintained at a therapeutic plasma concentration from at least 1 hour to at least 24 hours after initial administration. After administration, the initial therapeutic action takes effect within the first hour and lasts at least about 24 hours. This therapeutic effect remains relatively and substantially stable for the remaining period of 24 hours. The formulations can be used for treating depression and/or sleeping disorders.