This study is designed for adults and elderly individuals with major depressive disorder (MDD) and trouble sleeping, known as insomnia. We're testing a new treatment called *seltorexant*. Participants continue their usual antidepressants but add seltorexant or a placebo (a pill with no active medicine) to see if it helps. **MDD** is a severe depression that affects mood and daily life. **Insomnia** means having difficulty falling or staying asleep. You must have tried antidepressants called **SSRIs** (Selective Serotonin Reuptake Inhibitors) or **SNRIs** (Serotonin-Norepinephrine Reuptake Inhibitors) but still experience depression symptoms besides insomnia. **Key Points:** - The study involves taking a new medicine along with your current treatment. - Participants should be willing to attend study visits and follow procedures. - **Eligibility** requires specific age, health conditions, and treatment history. To join, you should not have severe health issues like kidney problems, heart disease, uncontrolled diabetes, or thyroid disorders. This study helps assess if seltorexant can improve depression and sleep issues safely. If you meet the criteria, this could be an opportunity to try a promising new treatment.