A Dual Orexin Receptor Antagonist to Reduce Biomarkers of Neurodegeneration in Adults With Insomnia.

Inclusion Criteria: * Diagnosis of insomnia disorder as defined by the DSM-5 (difficulty initiating or maintaining sleep or waking up too early for at least 3 nights per week, for at least 3 months, with adequate opportunity and circumstances for sleep and at least one daytime impairment related to the sleep difficulty) and a score ≥15 on the ISI. * Able to provide informed electronic consent. * Fluent English literacy. * Adults aged between 40-65 years. Exclusion Criteria: * People highly dependent on medical care as determined by a medical officer. * Untreated moderate-severe sleep apnoea as diagnosed using in-home wrist oximetry (oxygen desaturation index\>15, ongoing effectively treated sleep apnoea with insomnia will be allowed). * Circadian disorders, narcolepsy, severe restless legs syndrome, and REM sleep behaviour disorder or uncontrolled psychiatric disorders. * History of attempted suicide or current suicide ideation (indicated by a score \>0 on Q9 of the Patient Health Questionnaire-9) at pre-screening. * Objective cognitive decline measured by scoring ≤26 on the Montreal Cognitive Assessment (MoCA) * Regular shift work, jet lag or trans-meridian travel (over 2h time difference) in the past week before randomisation. * Pregnancy or lactation. Women will be advised to use contraception for the duration of the study and a urine pregnancy test will be performed when necessary. * Any other condition that would, in the Investigator's judgment, contraindicate the patient's participation in the clinical trial due to safety concerns or compliance with clinical study procedures. * Currently participating in or has participated in a research study of an investigational agent or device within 4 weeks of enrolment. * Concomitant use of medicines that are inhibitors (e.g., diltiazem, fluvoxamine, fluconazole, itraconazole, verapamil), or moderate to strong inducers of CYP3A4 (e.g., carbamazepine, modafinil, phenytoin, rifampicin, St John's Wort) * Ongoing use of anti-psychotic medication, bosentan, efavirenz, etravirine, modafinil, rifampin, carbamazepine or illicit stimulants. * Regular use of hypnotics (including melatonin, valerian, kava, benzodiazepines and Z-drugs), and other medications that can cause additive sedation (e.g. sedating antihistamines, tricyclic antidepressants, antipsychotics) within 14 days or 4-5 half-lives (whichever is longer) of starting the clinical trial. * Regular use of psychostimulants (e.g., dexamfetamine, lisdexamfetamine, methylphenidate) or non-amphetamine psychostimulants (e.g., armodafinil, modafinil, atomoxetine) within 14 days or 4-5 half-lives (whichever is longer) of starting the clinical trial. * Use of antidepressant medications for treatment of low mood for less than one year or dose changes (escalation or tapering) or change in antidepressant medications within the past year. * Regular use opioids within 14 days or 4-5 half-lives (whichever is longer) of starting the clinical trial. * Ongoing use of THC- or CBD-containing products within 14 days prior to the start of the trial. * Dependence or any other drug or alcohol dependence within the past two years (alcohol to be limited to no more than 2 standard drinks per day during trial period). * Allergy to lactose.