Shifts are started when the player pays their staff their pay. The player may choose the length of each shift. The longer the shift, the higher the wages for the employees are. When a shift is started a timer will appear at the bottom of the page. It's recommended to pay the longest shifts so you'll be able to start a next shift the next day without having a long wait for your customers.
See also Edit
These references come from the game itself, however, you need to go to http://apps.facebook.com/hotelcity, have Flash Player 10 installed, and have a Facebook account in order to prove the references.