In the past few years, a trend of opening zoos later or keeping them open longer has been spreading across the globe, especially during summertime. Recently, I saw that San Diego Wild Animal Park is offering night safaris until 9pm, which got me to wondering why all zoos dont capitalize on this. For one thing, allowing visitors to come at night opens your market up to young professionals who are busy working during weekdays, and uninterested in fighting the family crowd (and heat) during the weekends. The teenage market would also, more than likely, love extended hours as teenagers are constantly looking for places to hang out, and the zoo would be a "safe" and acceptable place from their parents point of view.
Increasing your market equals increased spending and increased chance to meet educational goals.
Secondly, nighttime visitation allows people to avoid the peak heat of summer days, and will theoretically increase their stay time. Additionally, many animals displayed in zoos are crepuscular (active during dawn and dusk) or nocturnal, and therefore, will be the most active during evening hours. Animal activity always equates increased stay times.
An increased stay time equals increased spending and increased chance to meet educational goals.
Opponents site the extended hours actually hurt the zoo due to increased cost of staffing for extra hours. However, some zoos in especially hot climates have actually changed their summer hours to shift toward the evening. For example, the Al Ain Zoo in Abu Dhabi, UAE changes their summer hours to open in the afternoon and stay open past midnight, when in winter, have more traditional morning through afternoon hours.
Another zoo in Singapore, the Singapore Night Safari Park, was actually developed to only be open in the evenings. Their novel approach has brought loads of attention from the industry and visitors worldwide, and from what I can tell, are tremendously successful. [youtube=http://www.youtube.com/watch?v=1Fd44p0Y_Ag&hl=en]