The ultimate aim of this study is to make detrended fluctuation analysis (DFA) useful for everyone. It introduces a practical method for making a device that can check cyclic rhythm in nature, such as the heartbeat. This book presents empirical evidence revealed by a modified DFA (mDFA). A heartbeat-checking algorithm, DFA, was made by Peng et al. in the mid-1990s. However, the technique has not been incorporated in a device for practical use. With a view to creating a device DFA, mDFA was made by modifying Peng's DFA--and a former graduate student (Katsunori Tanaka) created the program under supervision of the author. To verify mDFA, hundreds of cyclic phenomena were studied and recorded. Crustacean hearts have more than a 100-year history in biology and physiology. They are advantageous for conducting experiments such as isolated-heart electrophysiology. Crustacean hearts are thus good models for human hearts. DFA techniques are essentially just tools quantifying the state of the heartbeat and its control. This book does not describe details about DFA, but it does describe how mDFA works and it is useful for analyzing fluctuation of various cyclic behaviors such as heart movements. In conclusion, mDFA is a useful tool for studying the heart function in terms of biomedical technology, engineering and animal physiology. --