In human volunteers who have been continuously experiencing unsatisfactory sleep, effects of glycine ingestion (3 g) before bedtime on subjective sleep quality were investigated, and changes in polysomnography (PSG) during sleep were analyzed. Effects on daytime sleepiness and daytime cognitive function were also evaluated. Glycine improved subjective sleep quality and sleep efficacy (sleep time/in-bed time), and shortened PSG latency both to sleep onset and to slow wave sleep without changes in the sleep architecture. Glycine lessened daytime sleepiness and improved performance of memory recognition tasks. Thus, a bolus ingestion of glycine before bedtime seems to produce subjective and objective improvement of the sleep quality in a different way than traditional hypnotic drugs such as benzodiazepines.