From Philadelphia to Denver nocturnal visitors are learning what happens when the gates slam shut, the sun goes down and the moon rises over some of America's most well-known zoos.

"You get the zoo to yourself," said Jennifer Labows of the Philadelphia Zoo. "It is a unique experience for guests to see the animals at night." The Philadelphia Zoo has been running its Roars and Snores Overnight Programs for about 20 years. The most popular theme program is the Night Flight Overnight Program where children aged five to 12 sleep in the zoo's tree house.

The overnight stays are not only popular with young children. Teen programs are offered at many zoos for those young adults interested in the zoo industry. They are also a favorite venue for birthday sleepovers, family trips and with scout troops. "We customize the overnight to whatever badge the scouts are trying to complete that night," Labows said.