To develop potent and orally bioavailable melatonin receptor (MT<sub>1</sub> and MT<sub>2</sub>) agonists, a novel series of 5-6-5 tricyclic derivatives was designed, synthesized, and evaluated. The synthesized indeno[5,4-<i>d</i>][1,3]oxazole, cyclopenta[<i>c</i>]pyrazolo[1,5-<i>a</i>]pyridine, indeno[5,4-<i>d</i>][1,3]thiazole, and cyclopenta[<i>e</i>]indazole derivatives showed potent binding affinities for MT<sub>1</sub>/MT<sub>2</sub> receptors. Further optimization of these derivatives based on their metabolic stability in human hepatic microsomes revealed that (<i>S</i>)-<b>3b</b> ((<i>S</i>)-<i>N</i>-[2-(2-methyl-7,8-dihydro-6<i>H</i>-indeno[5,4-<i>d</i>][1,3]oxazol-8-yl)ethyl]acetamide) was a potent MT<sub>1</sub> and MT<sub>2</sub> ligand (MT<sub>1</sub>, <i>K</i><sub>i</sub> = 0.031 nM; MT<sub>2</sub>, <i>K</i><sub>i</sub> = 0.070 nM) with good metabolic stability in human hepatic microsomes. Moreover, compound (<i>S</i>)-<b>3b</b> showed good BBB permeability in rats, and its in vivo pharmacological effects were confirmed by its sleep-promotion ability in cats.