Arduino input fluctuates

With the following code I get a log like the one below. Why is that?

int analogPin = 5;
 int val = 0;
 
 void setup(void) {
    Serial.begin(9600);
 }
 
 void loop(void) {
    val = analogRead(analogPin);
    Serial.print("Analog reading = ");
    Serial.println(val);   
 }
 

Result:

Analog reading = 998
 Analog reading = 981
 Analog reading = 511
 Analog reading = 159
 Analog reading = 24
 Analog reading = 108
 Analog reading = 439
 Analog reading = 946
 Analog reading = 1023
 Analog reading = 420
 Analog reading = 116
 Analog reading = 25
 Analog reading = 151
 Analog reading = 542
 Analog reading = 997
 Analog reading = 982
 Analog reading = 513
 Analog reading = 161
 Analog reading = 25
 Analog reading = 107
 Analog reading = 437
 Analog reading = 945
 Analog reading = 1023
 Analog reading = 421
 Analog reading = 117
 Analog reading = 25
 Analog reading = 150
 Analog reading = 541
 Analog reading = 997
 Analog reading = 983
 Analog reading = 515
 Analog reading = 162
 Analog reading = 25
 Analog reading = 107
 Analog reading = 437
 Analog reading = 945
 Analog reading = 1023
 Analog reading = 422
 Analog reading = 117
 Analog reading = 25
 Analog reading = 149
 Analog reading = 540
 Analog reading = 997
 Analog reading = 983
 Analog reading = 516
 Analog reading = 162
 Analog reading = 25
 Analog reading = 107
 Analog reading = 436
 Analog reading = 945
 Analog reading = 1023
 Analog reading = 422
 Analog reading = 117
 Analog reading = 25
 Analog reading = 150
 Analog reading = 540
 Analog reading = 998
 Analog reading = 982
 Analog reading = 516
 Analog reading = 162
 Analog reading = 25
 Analog reading = 108
 Analog reading = 437
 

Is the result just noise?

And as soon as I plug in a cable I see the following pattern:

Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 72
 Analog reading = 447
 Analog reading = 1023
 Analog reading = 1023
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 118
 Analog reading = 849
 Analog reading = 1023
 Analog reading = 835
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 40
 Analog reading = 401
 Analog reading = 1023
 Analog reading = 1023
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 0
 Analog reading = 89
 Analog reading = 475
 Analog reading = 1023
 Analog reading = 1023
 

Is that my heartbeat or so??? No idea what can have such an influence on a empty pin.

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