From www.wdwinfo.com

Walt Disney World resorts to enforce pool hours
Sep 22, 2013

New rules will be in force for the pools at the Walt Disney World resorts beginning this November. Not all the pools are fenced in, so even when they are officially closed, guests are still able to swim, though there are no lifeguards overnight. Fences will be added around any pools that don't currently have them, so guests will only have access when there are lifeguards on duty. The pools will be closed from 11:00pm to 7:00am.

One possible reason for this change could be the drowning of a little boy in March in the Pop Century. The boy was swimming with relatives at 9:30 pm; lifeguards are only on duty until 8:00pm.