This clinical trial is focused on studying the effects of a medication called seltorexant on adults and elderly patients who have Major Depressive Disorder with symptoms of insomnia. Major Depressive Disorder, often referred to as depression, is a condition characterized by persistent feelings of sadness and loss of interest, which can significantly impact daily life. Insomnia is a common symptom of depression, where individuals have trouble falling or staying asleep.

The purpose of the study is to evaluate how well seltorexant works when used alongside other antidepressants, specifically those known as SSRIs (Selective Serotonin Reuptake Inhibitors) or SNRIs (Serotonin-Norepinephrine Reuptake Inhibitors). The study will compare the effects of taking seltorexant with a placebo, which looks like the medication but does not contain the active ingredient. The study aims to see if seltorexant can improve depressive symptoms and help maintain these improvements over time.

Participants in the study will take part in two main parts. In the first part, the effectiveness of seltorexant in improving symptoms of depression will be assessed. In the second part, the focus will be on whether seltorexant can help prevent the return of depressive symptoms after initial improvement. The study will involve taking the medication in the form of a film-coated tablet by mouth. The trial is designed to ensure the safety and effectiveness of seltorexant as an additional treatment for those who have not fully responded to their current antidepressant therapy.