This study is for adults and older people who have major depressive disorder with insomnia symptoms (MDDIS). It tests how well a medicine called **seltorexant** works when added to an antidepressant. **Seltorexant** could help improve depressive symptoms for people who haven't had enough improvement from their current antidepressants, such as SSRIs (Selective Serotonin Reuptake Inhibitors) or SNRIs (Serotonin-Norepinephrine Reuptake Inhibitors). Participants must have tried 1 or 2 antidepressants without complete success but have shown some improvement. They should be taking a stable dose of their current medication. The study excludes those with serious health conditions like recent kidney issues, thyroid problems, or certain other diseases.

• Participants will need to be on a stable antidepressant for at least 6 weeks before joining.
• People should be able to safely take the study medicine and follow the study rules.
• Participants with major health issues or specific conditions like narcolepsy or seizure history cannot join.