Error C99

In this code to test servos I get an error for the "sleep" function.

The error is: "warning: implicit declaration of function 'sleep' is invalid in C99.
What does that error refer to ??

int main() {
printf("Powering servo to its center position\n");
set_servo_position(3, 1000);
enable_servos();

sleep(4);

printf("Sending servo to position 200\n");
set_servo_position(3, 200);

sleep(4);

printf("Sending servo to position 1800\n");
set_servo_position(3, 1800);

sleep(4);

printf("Turning servos off\n");
disable_servos();
}