Matching placebo to seltorexant 40 mg capsules (over-encapsulated tablets) will be administered orally.

The mean SpO2 during TST on Night 4 and Night 1 will be assessed by continuous finger pulse oximetry.

The mean SpO2 during REM, NREM and awake stages of sleep will be assessed by continuous finger pulse oximetry.

The percentage of TST with SpO2 level less than 90%, 85%, and 80% during the night, assessed by continuous finger pulse oximetry will be reported.

LPS is a sleep parameter and will be measured by PSG during PSG screening. Elapsed time from the beginning of the PSG recording to the onset of the first 10 minutes of continuous sleep will be measured over 4 nights and the average time to sleep will be calculated.

WASO is a sleep parameter and will be measured by PSG during PSG screening. WASO is measured during overnight sleep laboratory PSG assessment and is defined as the duration of wakefulness from the onset of persistent sleep (that is, 10 consecutive minutes of sleep) over the first 6 hours of PSG assessment. The number of minutes in the Awake stage after the onset of persistent sleep to the end of the recording.

SE is a sleep parameter and will be measured by PSG during PSG screening. The total sleep time divided by the total time in bed (that is, the number of minutes from the beginning of the PSG recording to the end of the recording).

TST is a sleep parameter and will be measured by PSG during PSG screening. All of the hours of non-rapid eye movement (NREM) and rapid eye movement (REM) sleep, as measured by PSG, are summed to determine the TST.

REM sleep is a normal stage of sleep characterized by the rapid and random movement of the eyes. REM sleep latency is the time from sleep onset until first period of REM sleep. PSG recordings will be used to determine the time spent in different sleep stages (S1: light sleep, S2: light sleep, S3: deep sleep, and S4: REM sleep).

PSG recordings will be used to determine the time spent in different sleep stages (S1: light sleep, S2: light sleep, S3: deep sleep, and S4: REM sleep). It will be calculated as number of epochs scored as REM sleep divided by 2.

NREM sleep latency is the time from sleep onset until first period of NREM sleep. Polysomnographic recordings will be used to determine the time spent in different sleep stages (S1: light sleep, S2: light sleep, S3: deep sleep, and S4: NREM sleep).

Polysomnographic recordings will be used to determine the time spent in different sleep stages (S1: light sleep, S2: light sleep, S3: deep sleep, and S4: NREM sleep). It will be calculated as number of epochs scored as NREM sleep divided by 2.

An AE is any untoward medical occurrence in a participant who received study drug without regard to possibility of causal relationship. A serious adverse event (SAE) is an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly.

An SAE is any AE that results in: death, persistent or significant disability/incapacity, requires inpatient hospitalization or prolongation of existing hospitalization, is life-threatening experience, is a congenital anomaly/birth defect and may jeopardize participant and/or may require medical or surgical intervention to prevent one of the outcomes listed above. All SAEs and events of special interest including cataplexy (sudden, transient episode of muscle weakness accompanied by conscious awareness), sleep paralysis (the experience of not being able to move, react, or speak when falling asleep/awakening), and complex, sleep-related behaviors/parasomnias such as confusional arousals, somnambulism (sleep walking), sleep terrors, bruxism (teeth grinding), sleep sex, sleep related eating disorder, sleep behavior disorder, and catathrenia (Rapid Eye Movement [REM]-associated end-inspiratory apnea/breath holding) will be reported.

Number of participants with clinically significant abnormalities in the vital signs including temperature, pulse/heart rate, respiratory rate, and blood pressure will be reported.

Number of participants with clinically significant abnormalities in the physical examination including examination of height, body weight, and waist circumference will be reported.

Number of participants with clinically significant abnormalities in the ECG will be reported.

Blood samples for serum chemistry, hematology, and urinalysis will be collected for clinical laboratory testing.

C-SSRS is a clinician rated assessment of suicidal behavior and / or intent. Scale consists of 28 items in 4 sections: suicide behavior, actual attempts, suicidal ideation, and intensity of ideation. Suicidal ideation consists of 5 'yes/no' items: wish to be dead, non-specific active suicidal thoughts, active suicidal ideation with any methods (not plan) without intention to act, active suicidal ideation with some intent to act without specific plan, active suicidal ideation with specific plan and intent. Only items with yes responses are listed. Worsening of suicidal ideation indicates an increase in severity of suicidal ideation from baseline.

The B-L VAS is a patient-rated scale designed to assess the current level of sedation and consists of sixteen 100-milliliter (mm) VAS anchored by antonyms (Alert-Drowsy, Lethargic-Energetic). Scores are combined to form 3 mood factors: alertness, calmness, and contentedness. The score is measured from the left to a mark made on the line by the patient and ranges from 0 (alert) to 100 (drowsy).

The KSS is a patient reported assessment of level of drowsiness at the time of scale administration. This scale is focused mainly on the propensity to fall asleep and has a high validity in measuring sleepiness. It consists of a 9-point Likert scale with response options from: 1=very alert, 3=alert, 5=neither alert nor sleepy, 7=sleepy (but not fighting sleep), 9=very sleepy (fighting sleep).

The SDMT is a widely used, paper-and-pencil assessment of complex scanning and visual tracking, requiring elements of attention, visuoperceptual processing, working memory, and cognitive/psychomotor speed. The test is viewed as a robust screening test for adult neuropsychological impairment and has been used to demonstrate worse cognitive functioning in patients with MDD. The test includes a coding key consisting of 9 abstract symbols, each paired with a number ranging from 1 to 9. Following the key, the participant is presented with randomly ordered symbols and is required to write the number corresponding to each symbol as fast as possible. The number of correct substitutions within 90 seconds is recorded by examiner-administered cognitive test battery.

The TMT-B measures divided attention and executive function (tracking and sequencing). The participant is instructed to draw a line to connect a set of 25 consecutively numbered and lettered circles, alternating sequentially between numbers and letters (that is, 1-A-2-B). The participant is instructed to work as quickly as possible while still maintaining accuracy. The TMT-B has acceptable reliability; reliability coefficients have typically been reported as exceeding 0.65. The TMT-B is sensitive to cognitive decline associated with MDD.

The HVLT-R, a measure of verbal learning and memory, is a 12-item word list recall test. Administration includes 3 learning trials, a delayed recall (20-minute) trial, and a 24-word recognition list (including 12 target and 12 foil words). The test administrator reads instructions and word lists aloud, and records words recalled/recognized by the participant. Three learning trials are combined to calculate a total recall score learning, delayed recall, and recognition trials.