Polysomnography is the standard method to score the sleep stages or to determine the quality of sleep. During all night examination, subjects must attach complicate and numerous electrodes on their body to acquire the biosignals. They can influence participants\' sleep stage transition or sleep pattern, and the method seems intricate. In many researches, it is reported that autonomic nervous system (ANS) is varying with sleep stage transition and heart rate variability (HRV) is one of the indices which reflects the changes of autonomic nervous system. In this point of view, we can estimate the sleep quality by observing the HRV variation. In this study, to analyze the heart rate variability, we introduce a new system that can detect the heart beats of subjects during sleep by using bed installed load-cell sensors. The pressure to the sensor changes with the pulsation of the heart and we consider it as ballistocardiogram, the physical heart beat signal. To validate our system, we adopted this system for the 4 subjects with the polysomnography. The results show the LF/HF ratio of the heart rate, one of the reflection parameters of ANS, acquired from the system for each sleep stage. To validate the results, the HRV from electrocardiogram using Ag-AgCl electrodes will be compared.