PROBLEM TO BE SOLVED: To use characteristics of both a modified cosine curve model and a generalization additive model by using both models, and provide a more accurate biorhythm estimation method.SOLUTION: A biorhythm estimation method includes: (A) a process for measuring a value of biological information in a predetermined time； (B) a process for analyzing the measured biological information by a generalization additive model； (C) a process for analyzing the measured biological information by a cosine curve model； (D) a process for analyzing the measured biological information by a plurality of modified cosine curve models； (E) a process for calculating the minimum value, the maximum value, and the apex phase of a biorhythm curve from B； (F) a process for comparing the results of C and D with a value calculated in the process of (A) using Akaike's Information Criterion； and (G) a process for calculating a phase, an amplitude, and instability of biorhythms from these calculation results to estimate biorhythms.SELECTED DRAWING: Figure 4
