Why can we stay awake longer than we sleep ?
Sleep is the bodies way to repair and rest. In the same way the human body needs food, water, and oxygen to function properly, humans also need sleep to sustain life. Research has proven that a person who has not slept show "progressive and significant deficits in concentration, motivation, perception and other higher mental processes as the duration of sleep deprivation increased." Animal research, conducted with rats, with studied sleep deprivation ended in the death after two weeks.
All of that said, the amount of daily sleep differs from person to person. The pace which each individual goes through the four different stages of sleep determine the amount needed. Therefore, given that the human body has received enough restorative sleep, the body is able to stay functioning and awake until the body, again, needs repair and rest.