The goal of this clinical trial is to learn if Lemborexant works to treat residual insomnia in adults with depression that is being treated. It will also learn about how practical, tolerable, and effective Lemborexant is. The main questions it aims to answer are:

• Does Lemborexant help participants improve sleep and reduce insomnia symptoms?
• How practical is it to use Lemborexant (how many participants join, drop out, and follow the study rules)? How do participants feel about using it (based on surveys and interviews)?

Researchers will compare Lemborexant to a placebo (a look-alike substance that contains no drug) to see if Lemborexant works to treat residual insomnia in adequately treated major depressive disorder.

Participants will:

• Take Lemborexant or a placebo every day for 6 weeks (2 weeks at 5 mg then 4 weeks at 10 mg)
• Complete clinical assessments and in-person study visits
• Maintain a digital sleep diary and complete daily and weekly self-report ecological momentary assessments (EMAs)
• Use a wearable device which will be used to collect and monitor physiological data

Major Depressive Disorder (MDD) is often accompanied by persistent residual insomnia which does not resolve, even after adequate MDD treatment. Lemborexant is an orexin receptor antagonist involved in the regulation of sleep-wake cycles with a favourable safety profile compared to other sleep medications. There are currently no trials evaluating the feasibility and clinical efficacy of Lemborexant as a treatment for residual insomnia in patients with adequately treated MDD. This pilot study will investigate the feasibility, tolerability, and efficacy of Lemborexant in adults with adequately-treated MDD and residual insomnia.

Inclusion Criteria:

1. Aged 18 to 70 (inclusive), with a self-reported body mass index (BMI) between 19 and 30 kg/m2 (inclusive);
2. Meet criteria for primary MDD diagnosis without psychotic symptoms, as defined by the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5)2, and currently in a MDE, confirmed by the MINI International Neuropsychiatric Interview (MINI)3;
3. Have not failed more than 2 trials of antidepressant treatments in the current MDE, and have a history of adequate response (clinical outcome rating score of 1 or 2) to at least 1 antidepressant treatment during the current MDE as determined by the Antidepressant Treatment History Form-Short Form (ATHF-SF)4;
4. Are outpatients;
5. Did not take non-psychotropic or non-central nervous system (CNS) medications suspected to affect sleep-wake function for at least 4 weeks before starting the study.
6. Self-reported subjective total sleep time (sTST) ≤ 6.5 hours, subjective sleep onset latency (sSOL) ≥ 30 minutes, and subjective wake after sleep onset (sWASO) ≥ 45 minutes per night. Time spent in bed (either sleeping or attempting to sleep) must be between 7 and 10 hours per night. Self-reported regular bedtime (i.e., the time the participant gets in bed) between 21:00 and 01:00 and regular wake time (i.e., the time the participant wakes and does not go back to sleep) between 05:00 and 10:00;
7. Confirmation of current insomnia symptoms as determined from responses on the Sleep Diary completed on at least 7 consecutive mornings (minimum 5 of 7 for eligibility), such that sSOL ≥ 30 minutes on at least 3 of the 7 nights and/or sWASO ≥ 45 minutes on at least 3 of the 7 nights;
8. Confirmation of sufficient duration of time spent in bed, as determined from responses on the Sleep Diary on the 7 most recent mornings before the visit, such that there are no more than 2 nights with time spent in bed of duration < 7 hours or > 10 hours;
9. Confirmation of regular bedtime (i.e., the time the participant gets in bed) between 21:00 and 01:00 on at least 5 of the 7 preceding nights, and regular wake time (i.e., the time the participant wakes and does not go back to sleep) between 05:00 and 10:00 on at least 5 of the 7 preceding nights.
10. Have a medically responsible physician (family doctor or psychiatrist) during their enrollment and participation in the trial;
11. Current 17-Item Hamilton Depression Rating Scale (HAM-D-17)30 score ≥ 8 and reporting an insomnia score of ≥15 on ISI1;
12. Are able to understand and comply with the requirements of the study, as judged by the investigator(s);
13. Provide written informed consent before initiation of any study-related procedures;
14. Own a smartphone and have reliable access to the internet and a browser on which to complete questionnaires.

Exclusion criteria:

1. Have taken or participated in any clinical trial of lemborexant and other drugs with the same mechanism (e.g., daridorexant), regardless of treatment outcome;
2. Have any known sensitivity to lemborexant or their excipients;
3. A lifetime history (current or previous) of bipolar I or II disorder, schizophrenia, schizoaffective disorder, schizophreniform disorder, delusional disorder, or psychotic symptoms as determined by the MINI;
4. Women who are pregnant or lactating (documented by a positive beta-human chorionic gonadotropin [beta-hCG] or human chorionic gonadotropin [hCG] urine test with a minimum sensitivity of 25 IU/L or equivalent units of beta-hCG or hCG);
5. Women who are not using an approved and effective method of contraception or family planning during the study. For example, combined estrogen- and progestogen-containing hormonal contraception associated with inhibition of ovulation (oral, intravaginal, or transdermal), progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable, or implantable), intrauterine device, intrauterine hormone-releasing system, bilateral tubal occlusion and ligation, vasectomized partner, sexual abstinence, or two forms of contraception with any barrier method or oral hormones (ie.g., condom plus diaphragm, condom or diaphragm plus spermicide, oral hormonal contraceptives plus spermicide or condom).
6. Positive toxicology screening results;
7. If participating in psychotherapy, must have been in stable treatment for at least 3 months prior to entry into the study, with no anticipation of change in the frequency of therapeutic sessions or the therapeutic focus 4 weeks before screening and the entire duration of participation;
8. Have active suicidal intent as determined by a score of 3 (severe suicidality with a clear plan and/or intent) or 4 (very severe: suicidal attempts) on item #3 on the HAM-D-17;
9. Have had a course of electroconvulsive therapy or intravenous ketamine therapy in the current episode or any previous episode;
10. Medical history of insomnia associated with another sleep disorder or any condition known to impact sleep. This includes any lifetime diagnosis of sleep-related breathing disorder, periodic limb movement disorder, restless legs syndrome, nightmare disorder, sleep terror disorder, sleepwalking disorder, rapid eye movement (REM) behaviour disorder, narcolepsy, or comorbid nocturia that is causing or exacerbating insomnia;
11. STOP-Bang5 scores ≥ 5; International Restless Legs Scale (IRLS)6 scores ≥ 16,Epworth Sleepiness Scale (ESS)⁷ ≥ 11
12. Habitual naps 4 or more days a week, occurring in the late afternoon or evening;
13. Transmeridian travel across more than 3 time zones in the 2 weeks before screening, or between screening and study baseline, or plans to travel across more than 3 time zones during the study;
14. Used any modality of treatment for insomnia, including cognitive-behavioural therapy within 2 weeks before screening;
15. Excessive caffeine use, defined as consuming more than 400 mg of caffeine per day (approximately 4 cups of brewed coffee), or habitual consumption of caffeine after 6:00 p.m., which in the investigator's opinion may contribute to insomnia;
16. Reports habitually consuming more than 14 drinks containing alcohol per week (females) or more than 21 drinks containing alcohol per week (males);
17. Used prohibited prescriptions or over-the-counter concomitant medications, or used any medication or sleep aid with known effects on sleep within 2 weeks before screening;
18. Report a history of sleep-related violent behaviour, or sleep driving, or any other complex sleep-related behaviour (e.g., making phone calls, preparing and eating food);
19. Diagnosis of substance dependence or abuse within the last 3 months as determined by MINI3;
20. Have a concomitant major unstable medical illness, cardiac pacemaker, or implanted medication pump;
21. Have any significant neurological disorder or insult including, but not limited to: any condition likely to be associated with increased intracranial pressure, space occupying brain lesion, any history of seizure except a febrile seizure of infancy, cerebral aneurysm, Parkinson's disease, Huntington's chorea, multiple sclerosis, significant head trauma with loss of consciousness for greater than 5 minutes;
22. A history of risk factors for Torsade de Pointes (e.g., heart failure, hypokalemia, family history of long QT syndrome) or the use of concomitant medications that prolonged the QTcF interval;
23. Scheduled for major surgery during the study;
24. Have a clinical finding that is unstable or that, in the opinion of the investigator(s), would be negatively affected by the study medication or that would affect the study medication (e.g., diabetes mellitus, hypertension, unstable angina);
25. Have uncorrected hypothyroidism or hyperthyroidism. Subjects needing a thyroid hormone supplement to treat hypothyroidism must have been on a stable dose of the medication for 30 days prior to enrolment;
26. Have any other condition that, in the opinion of the investigator(s), would adversely affect the subject's ability to complete the study or its measures.
27. Non-English-speaking individuals because the ability to communicate study information, answer questions accurately and completely about the study, and obtain consent are necessary.

Data analyst and co-investigators