Safety and Efficacy Study in Primary Insomnia Patients-Study B (4305-029)

Inclusion Criteria:

• Must be >=18 yrs old on the day of signing informed consent
• Diagnosed with Primary Insomnia
• Patient is in good physical and mental health
• Patient >=65 yrs old score at least 25 on the Mini Mental State Exam
• A female patient who is of reproductive potential has a negative serum pregnancy test and agrees to use contraception
• Patient reports difficulty with initiating and maintaining sleep during the 4 weeks prior to Visit 1 (accordingly to specific protocol criteria)
• Patient reports spending 6.5 to 9 hours nightly in bed on at least 3 out of 7 nights prior to Visit 1
• The patient's regular bedtime is between 9 pm-1 am
• Patient is willing to refrain from napping while in study
• Patient is able to read, understand and complete questionnaires and all diaries
• Patient is willing to limit alcohol, caffeine, and nicotine consumption while in the study
• For a portion of patients: Patient must be willing to stay overnight in a sleep laboratory and must be willing to stay in bed for at least 8 hours each night while at the sleep laboratory

Exclusion Criteria:

• Female patient is pregnant and/or breastfeeding at Prestudy visit, or expecting to conceive while in study
• History or diagnosis of another sleep disorder
• Difficulty sleeping due to a medical condition
• History of a neurological disorder
• History of bipolar disorder, psychotic disorder, or posttraumatic stress disorder, or current psychiatric disorder that requires a prohibited medication
• Ongoing depression
• History of substance abuse or dependence
• History or current evidence of a clinically significant cardiovascular disorder or clinically significant ECG at Prestudy Visit
• Patient taking certain prohibited medications
• Patient consumes the equivalent of >15 cigarettes a day
• Patient has a history of malignancy <= 5 years prior to signing informed consent, with the exception of basal cell or squamous cell skin cancer if adequately treated
• Patient is considered morbidly obese
• Patient was previously randomized in another investigational study of MK4305