Wake up Adafruit Feather 32u4 from sleep mode with external interrupt - c

Question

I have an Adafruit Feather ATMega 32u4. I want to put it into sleep mode and wake it up with pressing a switch as an external interrupt. This is what I tried so far and which worked:
switching a LED on and off with the switch
putting the Adafruit to sleep and let it wake up with a watchdog timer.
I tried to combine the code examples that I found in the Internet to make the code fit my board. I manage to put it to sleep but then I have to reset it because the Interrupt doesn't work. I would be very happy for any help and please don't blame me, I am a complete beginner!
Here is my code:
#include <avr/sleep.h>
#include <avr/power.h>
#include <avr/interrupt.h>
int pin2 = 2;
void pin2Interrupt(void) {
/* This will bring us back from sleep. */
detachInterrupt(digitalPinToInterrupt(2));
}
void enterSleep(void) {
/* Setup pin2 as an interrupt and attach handler. */
delay(100);
set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable();
attachInterrupt(digitalPinToInterrupt(2), pin2Interrupt, RISING);
if (digitalRead(pin2)){
sleep_mode();
}
/* The program will continue from here
* First thing to do is disable sleep.
*/
sleep_disable();
}
void setup() {
Serial.begin(9600);
/* Setup the pin direction. */
pinMode(pin2, INPUT_PULLUP);
Serial.println("Initialisation complete.");
}
int seconds = 0;
void loop() {
delay(1000);
seconds++;
Serial.print("Awake for ");
Serial.print(seconds, DEC);
Serial.println(" second");
if(seconds == 3){
Serial.println("Entering sleep");
delay(200);
seconds = 0;
enterSleep();
}
}
Update:
#include "LowPower.h"
#include "avr/interrupt.h"
#define BUTTON 3
#define LEDPIN 13
void wakeUp() {
detachInterrupt(digitalPinToInterrupt(BUTTON));
}
void Blink(byte ledPin, int msDelay, byte loops) {
for (byte i=0; i<loops; i++) {
digitalWrite(ledPin, LOW);
delay(msDelay);
digitalWrite(ledPin, LOW);
delay(msDelay);
}
}
void setup() {
pinMode(BUTTON, INPUT);
digitalWrite(BUTTON, LOW);
pinMode(LEDPIN, OUTPUT);
digitalWrite(LEDPIN, LOW);
}
void loop() {
cli();
EIMSK &= ~(1 << INT0);
EICRA |= (1 << ISC01) | (1 << ISC00);
EIMSK |= (1 << INT0);
attachInterrupt(digitalPinToInterrupt(BUTTON), wakeUp, CHANGE);
sei();
LowPower.powerDown(SLEEP_FOREVER, ADC_OFF, BOD_OFF);
Blink(LEDPIN, 100, 3);
Serial.println("AWAKE!");
delay(100);
}