Use analogWrite() to fade an LED up and down.

Lighthouse.ino

int ledPin = 6; // Note that pin 13 on the Uno doesn't support PWM!
int brightness = 0;
int stepValue = 1;

void setup() {
  pinMode(ledPin, OUTPUT); // sets the pin as output
}

void loop() {
  
  analogWrite(ledPin, brightness);
  delay(10);
  
  brightness = brightness + stepValue;
  
  if ( brightness >= 255 ) {
     stepValue = -1;
  }else if ( brightness <= 0 ) {
     stepValue = 1; 
  }

}