Inclusion Criteria:

• are at least 65 years of age at screening;
• sign written informed consent after the scope and nature of the investigation have been explained to them, before screening evaluations;
• are able to speak, read and understand the language of the investigator, study staff (including raters) and the informed consent form, and possess the ability to respond to questions, follow instructions and complete questionnaires;
• have demonstrated capability to independently complete the LogPad questionnaires and have completed the questionnaires at least 6 out of 7 days of the week preceding randomization;
• have a regular sleep pattern, meaning bedtime regularly occurs between 2100 and 2400, with no more variation from these boundaries than 2 times/ week, with 5-8.5 hours in bed;
• have a documented diagnosis of chronic primary insomnia, defined as fulfillment of the DSM-IV-TR criteria for primary insomnia (DSM-IV-TR 307.42) with a duration of >= 1 month; fulfill the following PSG criteria on the two screening/baseline PSG nights:
• average TST < 6.5 h (and each night greater than or equal to 3 h and < 7 h),
• average WASO greater than or equal to 45 minutes (and each night greater than or equal to 30 min),
• average LPS 15 min (and each night greater than or equal to 10 min).

Exclusion Criteria:

• have other sleep disorders (DSM-IV-TR), such as sleep related breathing disorders (AHI greater than or equal to 15), periodic leg movements with arousals (PLMAI greater than opr equal to 10), restless leg syndrome, narcolepsy, circadian sleep wake rhythm disorders, REM behavioral disorder or any parasomnia;
• have any significant medical or DSM-IV-TR psychiatric illness causing the sleep disturbances;
• currently meet diagnostic criteria for DSM-IV-TR depression (MDD) or have been diagnosed and treated for MDD within the last 2 years;
• have a history of bipolar disorder, a history of suicide attempt or a family history of suicide. A family history of suicide is defined as any history of suicide in the first and second degree family (parents, siblings, grandparents, or offspring), or a pattern of completed suicides (more than one) in the third degree family (aunts, uncles, nieces and nephews);
• have a history or signs of dementia or other serious cognitive impairment, as defined by a score of less than 26 on the Mini-Mental State Examination;
• have a significant, unstable medical illness e.g. acute or chronic pain, hepatic, renal, metabolic or cardiac disease;
• had serious head injury or stroke within the past year, or a history of (non-febrile) seizures;
• have clinically relevant ECG abnormalities at screening, as judged by the investigator;
• have clinically relevant abnormal hematology or biochemistry values at screening, as judged by the investigator;
• have DSM-IV-TR substance abuse or DSM-IV-TR addiction within the last year;
• drink more than 2 alcoholic drinks in a day. One drink is approximately equal to: 12 oz or 360 ml of beer (regular or light), or 4 oz or 120 ml of red or white wine, or 2 oz or 60 ml of desert wine (e.g. port, sherry), or 12 oz or 360 ml of wine cooler (regular or light), or 1 oz or 30 ml or spirits (80 to 100 proof, e.g. whiskey, vodka);
• are routinely sleeping during daytime (napping) for more than 20 minutes per day, 3 days or more per week;
• are night workers or rotating shift workers currently, or in the past 6 months
• use of psychotropic drugs affecting sleep within two weeks prior to randomization (fluoxetine: five weeks);
• use of concomitant medication affecting sleep (e.g. anxiolytics, sedatives, antidepressants, antipsychotics, centrally active sedating antihistamines, CNS stimulants, alpha-2-antagonists, respiratory stimulants and decongestants);
• smoke > 15 cigarettes per day and/or can not abstain from smoking during the night;
• drink excessive amounts of caffeinated beverages/day (more than 500 mg caffeine per day);
• have a body mass index (BMI) = 36;
• have a positive urine drug screen at screening or at baseline;
• have a known hypersensitivity to mirtazapine or to any of the excipients;