Why does a census usually measure where people sleep rather than where they work?

A census measures where people sleep—their nighttime population—rather than where they work because the primary goal of a modern census is to enumerate every person within a precisely delimited territory to understand the basic structure and trends of a society.12

Whether a census is conducted on a de jure basis (counting people at their legal or regular residence) or a de facto basis (counting them where they are physically present at the time of enumeration), the methodology is designed to allocate individuals to a specific location for consistent reporting. By focusing on where people sleep, the census provides a stable, standardized geographic distribution of the population.1

In contrast, measuring where people work would capture a "daytime population," which often fluctuates significantly and does not reflect where individuals reside, access local services, or form households. Consequently, using nighttime residence as the standard ensures that the data accurately reflects the territorial distribution of the population for governmental and scientific purposes.12