We found in our previous study that glycine ingestion (3 g) before bedtime significantly produced subjective sleep effects in volunteers who had been continuously experiencing unsatisfactory sleep. Further, a series of published reports has shown that the safety of glycine is relatively high. The present study was conducted to assess acute adverse events or a daytime sleepiness effect after using a 3-fold higher dose (9 g) than the previous study. The results indicated that 9 g doses of glycine produced changes in several clinical test parameters, which were however within the range of physiological normal variation, and changes in blood level of some amino acids, which also were considered to be within or very close to the range of physiological normal variation. Digestive symptoms occurring only after the bedtime ingestion were observed; though these possibly were adverse events, they were not serious. Further, glycine did not produce daytime sleepiness. It could be concluded that 9 g of glycine produced neither acute serious adverse events nor a daytime sleepiness carry-over effect.