Final answer:
Circadian rhythms are a subset of biological rhythms, specifically referring to 24-hour cycles such as the sleep-wake cycle, and changes in heart rate, blood pressure, and body temperature.
Step-by-step explanation:
The relationship between biological rhythms and Circadian rhythms can best be expressed by stating that Circadian rhythms are a subset of biological rhythms. Biological rhythms are internal rhythms of biological activity that can vary in length, while circadian rhythms specifically refer to biological rhythms with a period of about 24 hours. For example, the human sleep-wake cycle, which adjusts according to the natural light-dark environment, is a clear instance of a circadian rhythm, as are daily variations in heart rate, blood pressure, and body temperature.