Change direction of the leds

Other Hardware LEDs and Multiplexing
3

To be more precise, i like the led strip to have the leds lighting up one after the other starting on 1 side of the strip, so its in the animation i think.
Like the sun coming up in the east and going down in the West.
I think i need to change the fill_solid command which lights up all the leds at once, to a command which fills the strip 1 led by one till all the leds are in on position.

Thanks

edit:

got home and made a sketch of the direction of the leds, which i was hoping to fit in the sunrise sketch.

#include "FastLED.h"

// Number of leds in 1 strip
#define NUM_LEDS 84

// Define the array of leds
CRGB ledstrip_1[NUM_LEDS];
CRGB ledstrip_2[NUM_LEDS];
CRGB ledstrip_3[NUM_LEDS];
CRGB ledstrip_4[NUM_LEDS];

void setup() 
{
 
    FastLED.addLeds<WS2812B, 4, GRB>(ledstrip_1, NUM_LEDS);
    FastLED.addLeds<WS2812B, 5, GRB>(ledstrip_2, NUM_LEDS);
    FastLED.addLeds<WS2812B, 6, GRB>(ledstrip_3, NUM_LEDS);
    FastLED.addLeds<WS2812B, 7, GRB>(ledstrip_4, NUM_LEDS);
}

void loop() {
  for ( byte i = 0; i < NUM_LEDS; i++ ) {
    ledstrip_1[i] = CRGB::Red;
    ledstrip_2[i] = CRGB::Red;
    ledstrip_3[i] = CRGB::Red;
    ledstrip_4[i] = CRGB::Red;
    FastLED.delay(1000);

  }
}