In the sleep laboratory study , polysomnography-derived latency to persistent sleep after middle-of-the-night was significantly improved for both genders at both 1.75 mg and 3.5 mg ZST ( females : 15.7 and 8.6 min , respectively , vs. 27.7 min [ placebo ] ; males : 19.0 and 12.7 min vs. 29.0 min [ placebo ] ) with no significant gender differences .