The Census Bureau has finally woken up to the fact that cities have different numbers of people in them during the day than they do at night. Traditionally, it has limited itself to counting people at their residence. Detailed data can be found here.
For example, the City and County of Denver's population is 28% higher in the day time, an increase of 155,486 people each day. Washington D.C., Boston and Seattle are the only cities of 500,000 or more people that experience a greater daily flux on a percentage basis. In contrast, Aurora, Colorado loses 50,827 people each day, and 18% decline, and the largest drop of a city of more than 250,000 people.
Colorado Springs gains about 21,000 people during the day, an increase of 5.8%. Greenwood Village, home to the Denver Tech Center where I currently work, gains 35,357 people during the day, an increase of 320.4% in its population. Only two other cities of population 5,000 or more have such a large percentage increase: Commerce City, California and Oak Brook Village, Illinois. Many suburbs, like Westminster, Thornton, Parker, Northglen, Lafayette, Federal Heights, Evergreen, Highlands Ranch, Ken Caryl and Arvada loose population during the daytime -- indeed, whether your population grows or declines during the daytime pretty much defines a suburb as either a bedroom community or a genuine urban center. Commerce City, Wheat Ridge, Sheridan, Golden, Greenwood Village and Englewood, as well as Denver, all gain population during the day.
One place where this makes a huge impact is crime statistics. Central cities appear to have higher crime rates than they actually do, because crime rates are based on residential population, but are driven by day time population. The large day time population of the District of Columbia, compared to its residential population, for example, is one reason for that city's phenomenally high crime rate (although not the only one).