Bio-rhythm refers to the natural patterns and cycles that exist within living organisms. There are a number of synonyms that can be used to describe these rhythms, including circadian rhythm, biological clock, internal clock, and body clock. Each of these terms refers to the cyclical nature of physical and mental processes, and the way in which they are regulated by the body's natural rhythms. Other potential synonyms for bio-rhythm might include natural rhythm, physiological rhythm, or inner rhythm. Ultimately, these terms all describe the same fundamental concept - the way in which our bodies and minds are attuned to the cyclical patterns of the natural world.