Next-day residual effects of single evening doses of 3 mg of eszopiclone, 7.5 mg of zopiclone, and placebo were assessed in a randomized, double-blind, placebo-controlled, 3-way crossover study that used a mild sleep restriction protocol (sleep duration, 7 hours). During each period, 91 healthy volunteers spent 2 consecutive nights in the laboratory with time in bed restricted to 7 hours. Volunteers completed the Continuous Tracking Test, Critical Flicker Fusion task, Digit Symbol Substitution Test, N-back tasks, and Linear Analogue Rating Scales every half-hour from 7.5 to 11.5 hours after dose, commencing 15 minutes after awakening. Nighttime dosing of both eszopiclone (3 mg) and racemic zopiclone (7.5 mg) was associated with next-day performance impairment, and these residual effects dissipated over time. Eszopiclone did not differ from zopiclone on the primary end point, mean Continuous Tracking Test tracking error averaged from 7.5 to 9.5 hours after dose; however, a prespecified post hoc parametric analysis of reciprocal-transformed data favored eszopiclone over racemic zopiclone (P = 0.026).