Inclusion Criteria:

• Be between the ages of 21 and 64 years, inclusive;
• Have a body mass index (BMI) between 21 and 34 kg/m2, inclusive;
• Be a primary insomniac as defined by DSM-IV criteria; in particular difficulty with both sleep initiation and sleep maintenance;
• Have subjective Latency to Sleep Onset > 45 minutes;
• Have a mean habitual subjective TST of <6.5 hours;
• Have a TST of 240 - 420 minutes at each of two baseline PSGs;
• Have a mean LPS > 20min, with neither value <15 minutes and mean WASO > 40 minutes on baseline PSGs;
• Be in good general health as determined by a thorough medical history and physical examination (including vital signs), 12-lead ECG and clinical laboratory tests;
• Have clinical laboratory values within normal reference range or not clinically significantly abnormal as judged by the Principal Investigator;
• Be off any investigational drug for at least 30 days prior to screening;
• If the patient is a female of childbearing potential, she must be using an acceptable method of contraception, must have a negative serum pregnancy test at screening, and must have a negative urine pregnancy test before randomization.
• Female patients who have been surgically sterilized or have had a hysterectomy are eligible if they have a negative pregnancy test at screening;
• Be able to read, understand, and provide written/dated informed consent before enrolling in the study, and must be willing to comply with all study procedures.

Exclusion Criteria:

• Clinically significant unstable medical illness;
• Evidence or history of clinically significant allergic (except for untreated, asymptomatic, seasonal allergies at time of dosing), hematological, renal, endocrine, pulmonary, gastrointestinal, cardiovascular, hepatic, psychiatric, or neurological disease;
• History of cancer other than basal cell carcinoma, squamous cell carcinoma of the skin or cancer in situ within 5 years of screening;
• Supine or sitting blood pressure > 140/90 mmHg at the screening or baseline visits;
• Heart rate >100 bpm at the screening or baseline visits;
• Within the past three years, clinically significant psychiatric illness, including chronic psychiatric illness or any Axis I condition;
• History or presence of chronic pain;
• History of epilepsy or serious head injury;
• Other than primary insomnia, a history of clinically significant sleep disorder including narcolepsy, parasomnia as an adult, circadian rhythm disorder, restless leg syndrome, or on the initial baseline PSG, sleep apnea [Apnea Hypopnea Index (AHI) >10] or Periodic Limb Movement Disorder [Periodic Limb Movement Arousal Index (PLMAI) > 10];
• Any condition that may affect drug absorption;
• Smokers who habitually smoke more than 10 cigarettes per day or smoke during the overnight hours;
• Travel across more than three time zones, an expected change in sleep schedules of 3 hours or more, or involvement in rotating shift work within 14 days prior to screening or during the study period;
• Self-report of napping ≥30 minutes more than 2 times per week within the last month;
• Any clinically significant abnormal finding on physical examination, vital signs, ECG, or clinical laboratory tests, as determined by the investigator. (The QTc interval must be ≤430 msec. for males and ≤450 msec for females);
• Known or suspected diagnosis of Acquired Immune Deficiency Syndrome, or previous or current positive result on human immunodeficiency virus antibody or antigen testing;
• History or laboratory finding of positive hepatitis B surface antigen or hepatitis C core antibody;
• History of allergies, or known sensitivity, hypersensitivity, or adverse reaction to any drug similar to adipiplon, including benzodiazepines, or zolpidem, zaleplon, zopiclone, or eszopiclone;
• Use of any psychotropic medications, including over-the-counter (OTC) and herbal products, that may affect sleep/wake function within one week or five half-lives (whichever is longer) prior to screening or need to use any of these medications at any time during the study;
• Pregnant or lactating females;
• Positive serum pregnancy test at screening or urine pregnancy test at baseline;
• Recent history (≤ one year) of alcohol or drug abuse, or current evidence of substance dependence or abuse as defined by DSM-IV criteria;
• Regular consumption of large amounts of xanthine-containing substances (i.e. more than 5 cups of coffee or equivalent amounts of xanthine-containing substances per day);
• Have eaten grapefruit or had grapefruit juice from baseline through the completion of study dosing;
• Self report of a usual consumption of more than 14 units of alcohol per week;
• Requiring the concomitant usage of any 3A4 inhibitors;
• Any prior exposure to adipiplon (formerly known as NG2-73).