To evaluate the antidepressant effect of adjunctive treatment with Eszopiclone in subjects receiving venlafaxine for the treatment of Major Depressive Disorder (MDD).

This is a double-blind, randomized, placebo-controlled, parallel-group study. The study consists of two groups of subjects with Major Depressive Disorder randomized to treatment with either eszopiclone 3 mg or placebo nightly at bedtime for 31 weeks beginning the night of Visit 2. In addition, all subjects will receive open-label treatment with venlafaxine. Safety and efficacy will be evaluated using clinical observations as well as physician and subject administered ratings scales. This study was previously posted by Sepracor Inc. In October 2009, Sepracor Inc. was acquired by Dainippon Sumitomo Pharma., and in October 2010, Sepracor Inc's name was changed to Sunovion Pharmaceuticals Inc.

Inclusion Criteria:

• Subjects must be male or female between the ages of 18 and 64, inclusive, at the time of signing consent.
• Subjects must have a primary diagnosis of Major Depressive Disorder (MDD) by DSM-IV criteria (296.XX) as determined by the Mini International Neuropsychiatric Interview (M.I.N.I.).
• MDD must be the condition that was chiefly responsible for motivating the subject to seek treatment.
• Subjects must have a 17-item Hamilton Depression Scale (HAM-D-17) total score of > 22 at the screening visit.

Exclusion Criteria:

• Subjects who have a HAM-D-17 total score < 18 at Visit 2 will be discontinued from the study.
• All subjects who do not meet DSM-IV criteria (307.42) for Insomnia related to a Major Depressive Disorder.
• Subjects with less than a total sleep time < 6.5 hours at least three times per week over the past month.