Ooops, forgot to include the code for this effect.

//Heartbeat LED for BrickPixel
// elCalvoMike 12-6-2008
// modified by Wayne R. Tyler 2021'11'09

// library for controlling the BrickPixel hardware (REQUIRED)
#include <Adafruit_NeoPixel.h>

 //analog pin location for ArduinoMini - change for other boards
#define LED LED_BUILTIN

// The BrickPixel equivalent of ledPin
Adafruit_NeoPixel pixels(3, 7, NEO_RGB + NEO_KHZ800);

int i = 0;
int pmw = 255; //set PWM max - this can differ for other board pins
int rate = 25; //this is the beats per minute (60000 ms)

//because there are two beats to simulate the ‘lub-dub’ of the heart,
// a 60 beat heart rate is only a value of 30 in the rate variable
//the delay is the key to this programs realism - divide the rate into a minute, then weight it and divide by the pmw
//you can modify the weight by changing the fractions (i.e .1, .2, .6) but to keep the timing correct, they should total 1
//.1+.2+.1+.6 = 1

void setup() {
    pinMode( LED, OUTPUT );

    // Initialize the NeoPixel object (this is REQUIRED)
    pixels.begin();

    // Clear out any prior BrickPixel settings
    pixels.clear();
}

// changed pixels.Color(i, i, i); to pixels.Color(i/5, i/5, i/5) so
//   BrickPixel LEDs didn't blast you with light.
void loop(){
    for( i = 0; i < pmw; i++ ) {
        analogWrite( LED, i );
        pixels.setPixelColor( 0, pixels.Color(i/5, i/5, i/5) );
        pixels.setPixelColor( 1, pixels.Color(i/5, i/5, i/5) );
        pixels.setPixelColor( 2, pixels.Color(i/5, i/5, i/5) );
        pixels.show();
        delay( ((60000/rate)*.1)/pmw );
    }

    for( i = pmw; i > 0; i-- ) {
        analogWrite( LED, i );
        pixels.setPixelColor( 0, pixels.Color(i/5, i/5, i/5) );
        pixels.setPixelColor( 1, pixels.Color(i/5, i/5, i/5) );
        pixels.setPixelColor( 2, pixels.Color(i/5, i/5, i/5) );
        pixels.show();
        delay( ((60000/rate)*.2)/pmw );
    }

    for( i = 0; i < pmw; i++ ) {
        analogWrite( LED, i );
        pixels.setPixelColor( 0, pixels.Color(i/5, i/5, i/5) );
        pixels.setPixelColor( 1, pixels.Color(i/5, i/5, i/5) );
        pixels.setPixelColor( 2, pixels.Color(i/5, i/5, i/5) );
        pixels.show();
        delay( ((60000/rate)*.1)/pmw );
    }

    for( i = pmw; i > 0; i-- ) {
        analogWrite( LED, i );
        pixels.setPixelColor( 0, pixels.Color(i/5, i/5, i/5) );
        pixels.setPixelColor( 1, pixels.Color(i/5, i/5, i/5) );
        pixels.setPixelColor( 2, pixels.Color(i/5, i/5, i/5) );
        pixels.show();
        delay( ((60000/rate)*.6)/pmw );
    }
}