OBJECTIVES: To find evidence that sleep is necessary for normal brain function, thus indicating that declines in both sleep quality and quantity are related to worse performance of many daily tasks and deteriorated physical functions. The present study investigates the relationships of balance control and reaction time with sleep quality. METHODS: 58 healthy (male 20, female 38) volunteers with informed consent participated in this study. The Self-reported Pittsburgh Sleep Quality Index (PSQI) was used to evaluate sleep quality and relevant factors, and the subjects were divided into groups A (PSQI < 5) and B (PSQI ≥ 5) based on this index. Static balance control and reaction time to visual stimuli were conducted to assess their relationship with sleep quality. RESULTS: Group B exhibited excessive daytime sleepiness significantly more often compared to group A. Static balance control did not markedly change relative to sleep quality, but reaction time and error to visual stimuli were significantly increased in group B compared to group A. CONCLUSION: These findings indicate that a decline in sleep quality can result in delayed reactions, as well as decreased accuracy of these reactions. They also suggest that low sleep quality may be associated with changes in physical functions, including balance control through reduced selective attention.