A Study to Investigate the Effects of ACT-541468 on Nighttime Respiratory Function in Patients With Mild to Moderate Obstructive Sleep Apnea

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.