The majority of bicycles sold in Chicago are not legal to ride at night. Unless a bike is equipped with a light you may not ride it at night. By my observation, very few bikes sold in the city come with a light. Reflectors are not enough, and having them does not place the rider in compliance with the law. Specifically, Chicago Municipal Code requires bicycles to have a white light on the front that can be seen from at least 500 feet. On the back a red light or reflector is required. Recently, the City of Chicago created a good (though slightly weird) video that demonstrates why it is so important to ride with a light at night:
Before you leave the bike store with your new ride, buy a light. If you don't have a light on your current ride, get one. Hell, go ahead and buy two, one for the front and one for the back. If you don't know what kind of light to buy here's a suggestion.