Purpose: The objective of this study was to investigate the initial effects of the vertical thickness of occlusal splints on the electromyographic (EMG) activities (integrated EMG values) of temporal and masseter muscles during sleep in subjects with a nocturnal bruxism habit using a portable EMG recorder. Materials and Methods: The subjects consisted of 12 volunteers (4 men and 8 women, average age of 25.3 years). All subjects had never worn splints before and had a habit of nocturnal bruxism. Two types of splint were made for every subject: a splint with a 3-mm vertical thickness at the central incisors (S3) and a splint with a 6-mm vertical thickness (S6). The muscle activities of the left anterior temporal muscles and masseter muscles were recorded without occlusal splints (NS), with the S3 splint, and with the S6 splint by a portable EMG recorder. The integrated EMG values were calculated to examine muscle activities under the 3 different conditions. Results: The integrated EMG values of masseter and temporal muscles decreased following insertion of the S3 splint but were not significantly affected by the S6 splint. Six subjects in masseter EMG and 7 subjects in temporal EMG got worse with the S6 splint compared to NS. Conclusions: When the occlusal splints were used as a bruxism countermeasure, it was suggested from the analysis of muscle activities during sleep at night that the S3 splint was superior to the S6 splint.