The hypnotic efficacy of gaboxadol, a selective extrasynaptic GABA A agonist (SEGA), was evaluated in a phase-advance model of transient insomnia. Healthy subjects (18-64 years) completed a randomized, double-blind, parallel group study in which the sleep period was advanced 4h from habitual sleep time. Polysomnographic (PSG) and self-reported sleep measures were used to compare gaboxadol 10mg (N =271) and 15 mg (N =274) versus placebo (N =277). In the placebo group, the phase-advance procedure disrupted sleep maintenance as measured by PSG wakefulness after sleep onset (WASO) and self-reported WASO (sWASO), and also, to a lesser extent, disrupted sleep onset as measured by PSG latency to persistent sleep (LPS) and self-reported time to sleep onset (sTSO). Both doses of gaboxadol decreased WASO and sWASO versus placebo (p