• AHI during TST after single-dose administration [ Time Frame: After single-dose (i.e., Night 1) administration (duration: 8 hours PSG recording) ]
• Mean SaO2 during TST [ Time Frame: After single-dose (i.e., Night 1) administration (duration: 8 hours PSG recording) ]
• AHI during the awake, non-REM, and REM phases [ Time Frame: After single-dose (i.e., Night 1) administration (duration: 8 hours PSG recording) ]
• Mean SaO2 during the awake, non-REM, and REM phases [ Time Frame: After single-dose (i.e., Night 1) administration (duration: 8 hours PSG recording) ]
• Percentage of TST during which SaO2 is < 90%, < 85%, and < 80% [ Time Frame: After single-dose (i.e., Night 1) administration (duration: 8 hours PSG recording) ]
• Change from baseline to EOP of both periods in subjective next-day performance (morning sleepiness, daytime alertness, and daytime ability to function) as assessed by VAS [ Time Frame: After single-dose (i.e., Night 1) and multiple-dose (i.e., Night 5) administration (duration: 2 x 8 hours PSG recording) ]
• Objective sleep parameters LPS, WASO, TST, and SEI following single- and multiple-dose administration, as measured by PSG [ Time Frame: After single-dose (i.e., Night 1) and multiple-dose (i.e., Night 5) administration (duration: 2 x 8 hours PSG recording) ]
• Safety profile including incidence of adverse events (AEs) and serious adverse events (SAEs) [ Time Frame: From baseline to EOS (duration: for up to 10 weeks) ]

• Drug: ACT-541468
  Film-coated tablet for oral use
• Drug: Placebo
  Film-coated tablet for oral use

• Experimental: Treatment A: ACT-541468
  50 mg once daily from Day 1 to Day 5 of Period A
  Intervention: Drug: ACT-541468
• Placebo Comparator: Treatment B: Placebo
  Matching placebo once daily from Day 1 to Day 5 of Period B
  Intervention: Drug: Placebo

Inclusion Criteria:

• Signed informed consent in a language understandable to the subject prior to any study-mandated procedure.
• Male and female subjects aged ≥ 18 years at Screening.
• Women of childbearing potential must have a negative serum pregnancy test at Screening and a negative urine pregnancy test at Screening Visit 2 and on Day 1 pre-dose of the first period. They must agree to consistently and correctly use a highly effective method of contraception with a failure rate of < 1% per year.
• Women of non-childbearing potential, i.e., postmenopausal, with previous bilateral salpingectomy, bilateral salpingo-oophorectomy or hysterectomy, or with premature ovarian failure, XY genotype, Turner syndrome, and/or uterine agenesis.
• Diagnosis of OSA according to the International Classification of Sleep Disorders documented by medical history and confirmed in a sleep laboratory in the context of diagnosing OSA within the last 3 years.
• Patient with mild to moderate intensity of OSA determined during OSA diagnosis and confirmed on the screening night PSG and defined as apnea/hypopnea index (AHI) ranging from 5 to < 30.

Exclusion Criteria:

• Any circumstances or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the protocol.
• Pregnant or lactating women.
• Modified Swiss Narcolepsy Scale total score < 0 at Screening, or history of narcolepsy or cataplexy.
• Subjects with clinically significant abnormality on the screening night PSG as per investigator judgment, including evidence of severe insomnia (i.e., sleep time < 5 h) periodic limb movement disorder with arousal index ≥ 10/h, restless legs syndrome, circadian rhythm disorder, REM behavior disorder, parasomnia including nightmare disorder, sleep terror disorder, and/or sleepwalking disorder.
• Need for continuous positive airway pressure device or a dental appliance device within the preceding 7 days prior to Screening Visit 2 and during the course of the study, i.e., from Screening Visit 2 to EOS.
• Evidence of any other clinically significant active pulmonary disease such as chronic obstructive pulmonary disease (COPD) (as per Global Initiative for Obstructive Lung Disease), based on investigator's judgment.
• History of surgical intervention for obstructive sleep apnea, except nose surgery.
• SaO2 < 90% during wakefulness or mean non-apneic SaO2 (i.e., outside events of apnea/hypopnea) < 85% for > 5 consecutive min during the screening night PSG.