An analyzing method is to be implemented in an analyzing device, and includes: (A) dividing respective cycle lengths of a plurality of cardiac cycles of a user during sleep into a plurality of groups based on an order of occurrence of the cardiac cycles； (B) for each group, performing Fourier transform on the cycle lengths of the group to generate a respective spectrum； (C) for each group, calculating total power of the respective spectrum； and (D) determining, for each group, whether sleep quality of the user in a time period during which the corresponding cardiac cycles occurred is poor based on the total power of the respective spectrum and a predetermined threshold.一種分析方法、裝置及系統，該方法藉由該裝置實施，並包含：接收對應一使用者的多個心跳週期，該等心跳週期是藉由該使用者在睡眠時所量測而獲得；以一時間切割區間，將該等心跳週期依照時間順序，區分為一第1區間、一第2區間、…及一第N區間，N為大於1的正整數；對該第i區間的該等心跳週期作快速傅立葉轉換，以產生對應的N個頻譜，i=1、2、…、N；計算該N個頻譜之每一者的一總功率；及判斷該N個總功率之其中小於一預設閥值者所對應的所在時間是該使用者處於睡眠品質不佳的狀態。
