The study is a single centre, randomised, double blind, placebo controlled 2-way crossover design in a group of 32 male and female primary insomnia patients. The medications under investigation are eszopiclone and placebo. Patients will receive the study medications and placebo on the evening of Day 2 of each treatment period. Performance will be assessed on Day 3 of each treatment period. Each treatment period will be separated by at least a 7-day washout period. 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.