The instantaneous effect of the sound of a passing truck on sleep of five male students, 23 to 25 years of age, was assessed. The peak levels of noise were adjusted to 55, 60, and 65 dB (A) with intervals of 18 and 13 minutes, and a frequency of one per hour. Background noise in the experimental room was Leq 42 dB (A). The sleep stage of each epoch was visually judged based on the criteria of Rechtschaffen & Kales. Data for pre-exposure epochs of sleep stage 2 and rapid eye movement (REM) were collected and 18 epochs, including two under noise exposure, were evaluated. Three sets of polygraphic data were sampled as controls from 10 minutes after 65 dB (A) exposure to the next exposure. Compared with the control, the percentage of stage 2 to shallower stages (stage 1, waking, or movement time (MT)) was significantly increased by exposure to 55, 60, and 65 dB (A). The change lasted for one minute with exposure to 55 dB (A), and 2 to 3 minutes with 60 and 65 dB (A). A decrease in the percentage of stage 2 to deeper stages was observed at 2 to 4 minutes after exposure to 60 and 65 dB (A). The percentage of stage 2 to MT increased at 60 and 65 dB (A). The percentage of stage REM to other stages increased with the exposure to 60 or 65 dB (A) within 4 minutes. The threshold of instantaneous change of stage 2 to shallower stages due to the sound of a passing truck was at the peak level at less than 55 dB (A), and that of stage REM to other stages at 55 to 60 dB (A).