Knight-Rider LED Program with Serial Monitor on/off switch

I've set up a series of LEDs in a line, and set up the code that will light these LEDs in sequence in 'knight-rider' style. I've tested the code that runs the loop, and it works perfectly.

Now to take it a step further, my goal in this code is to be able to turn on/off the LED loop using the serial monitor. When I enter '1' into the serial monitor, the LED loop should begin and continue until I enter '0' into the serial monitor to turn the loop off.

The problem I am having with this code; when I enter '1' into the serial monitor, the knight-rider LED loops only once and then stops. I haven't been able to figure out how to keep the loop running continuously until it receives a '0' to turn it off. Any suggestions would be greatly appreciated.

int timer = 100;           // The higher the number, the slower the timing.
int incomingByte = 0;          // variable to read incoming serial instruction

void setup() {

  for (int thisPin = 2; thisPin < 6; thisPin++)  {
    pinMode(thisPin, OUTPUT);      
  }
  Serial.begin(9600);
}

void loop() {
  // check the serial port for incoming data
  if (Serial.available() > 0) {
    incomingByte = Serial.read();
  }
  // if it receives an 'on', then begin the LED knight-rider loop
  if (incomingByte == '1'){
    // loop from the lowest pin to the highest:
    for (int thisPin = 2; thisPin < 6; thisPin++) {
      // turn the pin on:
      digitalWrite(thisPin, HIGH);  
      delay(timer);                  
      // turn the pin off:
      digitalWrite(thisPin, LOW);    
    }  
    // loop from the highest pin to the lowest:
    for (int thisPin = 5; thisPin >= 2; thisPin--) {
      // turn the pin on:
      digitalWrite(thisPin, HIGH);
      delay(timer);
      // turn the pin off:
      digitalWrite(thisPin, LOW);
    }
  }
  if (incomingByte == '0'){  //if it receives a '0', turn off the system - do nothing
  }
}