The correct answer is option 3 - biological clock. Circadian rhythm refers to the natural, internal process that regulates the sleep-wake cycle and other biological processes in living organisms, including humans. It is often referred to as the "biological clock" because it controls the timing and duration of various physiological and behavioral functions.

Circadian rhythms are governed by an internal biological clock that is located in the brain and is sensitive to changes in light and darkness. This clock helps to regulate patterns of sleep and wakefulness, as well as other functions such as appetite, hormone production, and body temperature.

Demography (option 1) refers to the study of populations and their characteristics, and it is not directly related to circadian rhythms. Economic growth (option 2) is the increase in the production and consumption of goods and services in an economy, and it is also not directly connected to circadian rhythms. Climate of a region (option 4) can influence circadian rhythms to some extent, as changes in daylight and temperature can impact sleep patterns, but it is not the primary factor.

Therefore, the correct answer is option 3 - biological clock.