Many studies have shown that sleepiness is a major cause of traffic accidents. Some occupations, such as airplane pilots and automobile drivers, are not suitable for patients with daytime sleepiness. Therefore, it is important to establish an objective assessment of datime sleepiness for screening commercial drivers or pilot and for diagnosis of obstructive sleep apnea. In this study we introduced a portable system and compared it with traditional polysomnography in evaluation of daytime sleepiness. Eight patients with OSA and eight normal subjects were studied. Sleep latency as judged by portable device was the same as that judged by conventional polysomnography and sleep latency measured from patients with OSA (18.12±9.14 minutes) was significantly shorter than that in normal subjects (> 40minutes) , p<0.01. In conclusion, portable device is as useful as conventional polysomnography in assessment of daytime sleepiness.