Almorexant in Primary Insomnia

Inclusion Criteria: * Men or women 18 - 65 years of age (inclusive). * Women of childbearing potential must have a negative urine pregnancy test at the screening visit, the screening adaptation night, and pre-treatment and use a reliable method of contraception during the entire study duration and for at least 3 months after study drug intake. Reliable methods of contraception are: * Barrier type devices (e.g., female condom, diaphragm, contraceptive sponge) only in combination with a spermicide. * Intra-uterine devices. * Oral, injectable, implantable or transdermal contraceptives only in combination with a barrier method. * Abstention, rhythm method, and contraception by the partner alone are not acceptable methods of contraception. Women not of childbearing potential are defined as prepubescent, postmenopausal (i.e., amenorrhea for at least 1 year), or surgically or naturally sterile. * Body mass index (BMI) between 18 and 30 kg/m2 (limits included) at screening visit. * 12-lead ECG without clinically relevant abnormalities at screening visit. * Hematology and biochemistry test results not deviating from the normal range to a clinically relevant extent at screening visit and following the screening/adaptation night. * Primary insomnia by DSM-IV-TR criteria based on medical history and the assessments performed at screening visit. * History of the following for at least 3 months prior to the screening visit: * Usual reported subjective total sleep time (TST) 3 - 6 hours. * Usual sleep disturbance with a subjective sleep onset latency of \> 30 min. * Daytime complaints associated with poor sleep (e.g., fatigue, irritability, difficulty concentrating). * Polysomnography (PSG) at screening/adaptation night confirming TST \< 6 h and LPS ≥ 20 min. * Willingness to refrain from CNS-active drugs for 5 half-lives of the respective drug (but at least 1 week) prior to the screening/adaptation night and up to the end of treatment period 2. The usage of short-acting hypnotics (defined as hypnotics with a half-life of up to and including 10 hours) is allowed up to 48 hours prior to each PSG night, i.e., prior to the screening/adaptation night and prior to the treatment PSG nights. * Urine drug test negative for barbiturates, cannabinoids, amphetamines, and cocaine at screening visit 1, screening/adaptation PSG night and pre-treatment. Urine drug test negative for benzodiazepines and opiates at screening/adaptation PSG night and pre-treatment. * Signed informed consent prior to any study-mandated procedure. Exclusion Criteria: * Symptom assessment questionnaire (SBB) for diagnosis of apnea resulting in a score \> 2 at screening visit. * Zung self-rating depression scale (SDS) and/or Zung self-rating anxiety scale (SAS) resulting in a raw score ≥ 50 at screening visit. * Restless legs syndrome and/or meeting all four essential diagnostic criteria for RLS (see Appendix 10). * Insomnia due to sleep apnea or periodic limb movement disorder as assessed by PSG at screening/adaptation night: * apnea/hypopnea index (AHI) \> 10/h * periodic limb movement arousal index \> 10/h * Major depressive disorder, severe psychosis, or significant anxiety disorder. * Pregnancy or breast-feeding. * Systolic blood pressure \> 160 mmHg and/or diastolic blood pressure \> 100 mmHg at screening visit. * Within the 2-month period prior to the screening visit, clinical evidence of alcoholism or drug abuse. * Any known factor or disease that might interfere with treatment compliance, study conduct or interpretation of the results such as psychiatric disease or a disease which may affect the pharmacokinetics of the study drug. * Treatment with strong inhibitors of CYP3A4 (e.g., azole derivatives, ritonavir, clarithromycin) within 1 week prior to the screening/adaptation PSG night and up to the end of treatment period 2. * Excessive caffeine consumption (regular caffeine consumption of \> 7 units per day). * Night shift workers. * Known hypersensitivity to any excipients of the drug formulation. * Planned treatment or treatment with another investigational drug within 1 month prior to randomization and up to the end of treatment period 2. * Known concomitant life-threatening disease with a life expectancy \< 24 months. * Unstable medical abnormality, significant medical disorder or acute illness. * Recruitment of the same patient twice to the same dose level. Patients may be recruited to a lower dose level, provided that there are at least 28 days between last study drug administration and screening/adaptation PSG night.