In this study, we have developed a new practical system for estimating circadian rhythm by using smart wear that can measure electrocardiogram (ECG) during sleep. This system can estimate the time and heart rate (HR) value to reach the lowest point in circadian rhythm. We show the system in detail. And for further application, we conducted the experiment for showing the effects of jet lag on the circadian rhythm by using the developed system. The results showed that the time of the lowest HR shifted earlier and the lowest HR was higher in case of traveling in a westward direction.