Background: Bruxism has been widely discussed. The concepts of normobruxism and pathobruxism have recently been proposed. Severe pain in the masticatory muscles, which may be caused by pathobruxism, needs to be addressed. However, a clear causal relationship between bruxism and chronic pain in the masticatory muscles has not been elucidated. In addition, the effects of clenching and grinding on the body and their differences remain unclear. In particular, no studies have examined the effects of bruxism-like jaw movements on the peripheral tissues of the masticatory muscles and the autonomic nervous system.This study aimed to investigate the effects of bruxism on the masseter muscles and autonomic nervous system and to examine the differences between clenching and grinding. Methods: A total of 18 healthy volunteers, including 8 men (age: 31.3 ± 2.9 years) and 10 women (age: 28.9 ± 3.2 years) who had consented to participate in the study. We experimentally instructed them to clench and grind; measured the dynamic changes in oxygenation, electromyography, and heart rate variability (HRV); and assessed the subjective degree of fatigue. Results: Oxygenation, clenching, and tissue oxygen saturation (StO 2 ) and oxyhemoglobin (OxHb) levels decreased, whereas deoxygenated hemoglobin (dOxHb) levels increased. No change in grinding occurred; however, significant differences in muscle usage and subjective fatigue were observed between clenching and grinding. No significant differences were identified in HRV. Conclusions Tissue oxygenation differed significantly between clenching and grinding. Clenching resulted in a significant ischemic state, which rapidly recovered after clenching stopped. Grinding did not affect tissue oxygenation.