This study is designed to find the optimal (best) doses of GW679769 that could be given daily to promote sleep onset and maintain sleep during the night without significant next-day thinking impairment and drowsiness in subjects with primary insomnia.

Inclusion Criteria:

• Difficulty going to sleep and/or staying asleep for at least the past 3 months.
• Insomnia must result in significant distress or impairment in functioning at home, socially or at work.
• Otherwise good health with no significant or unstable medical disorder including psychiatric, neurological, endocrine, heart, lung or gastrointestinal disorders, drug/alcohol abuse, cognitive impairment, cancer or chronic pain conditions such as arthritis.

Exclusion Criteria:

• History of other sleep disorders such as sleep apnea or restless leg syndrome.
• Regular sleep habits, including bedtime between 9 PM and midnight.
• No nightshift/rotating shift work, frequent napping or planned travel across >2 time zones.
• Use of nicotine, caffeine and alcoholic products.