#include <M5StickCPlus.h>

//seconds
const int timeout = 10;

hw_timer_t * timer = NULL;
bool go_to_sleep = false;

void IRAM_ATTR onTimer()
{
  Serial.println("onTimer called");
  go_to_sleep = true;
}

void setup() {
  sleep_wake();
  M5.begin();
  M5.Lcd.fillScreen(BLACK);
  set_backlight();
  setCpuFrequencyMhz(80);
  timer = timerBegin(0, 80, true);
  timerAttachInterrupt(timer, &onTimer, true);
  timerAlarmWrite(timer, timeout * 1000 * 1000, true);
  timerAlarmEnable(timer);
  M5.Lcd.setCursor(0, 35, 4);
  battery_display();
}

void loop() {
  delay(1000);
  sleep_check();
}

void sleep_check(void)
{
  if(go_to_sleep == true) {
    deep_sleep();
  }
}

void set_backlight(void)
{
    M5.Axp.ScreenBreath(8);
}

void clear_txt(void)
{
  M5.Lcd.fillScreen(BLACK);
}

void battery_display(void)
{
  M5.Lcd.setCursor(20, 20, 4);
  M5.Lcd.setTextSize(2);
  M5.Lcd.printf("%d%%", (int) battery_level());
  M5.Lcd.print("\n");
  M5.Lcd.setTextSize(1);
  M5.Lcd.printf("%fV", M5.Axp.GetBatVoltage());
}

double battery_level(void)
{
  uint16_t vbatData = M5.Axp.GetVbatData();
  double vbat = vbatData * 1.1 / 1000;
  double percentage = 100.0 * ((vbat - 3.0) / (4.07 - 3.0));
  return min(percentage, 100);
}

void sleep_wake(void)
{
  pinMode(GPIO_NUM_37, INPUT_PULLUP);
  esp_sleep_enable_ext0_wakeup(GPIO_NUM_37, LOW);
}

void backlight_off(void)
{
  delay(1000);
  M5.Axp.ScreenBreath(0);
}

void deep_sleep(void) {
  clear_txt();
  M5.Lcd.setCursor(0, 30, 2);
  M5.Lcd.printf("Deep sleep\n");
  delay(1000);
  clear_txt();
  backlight_off();
  delay(3900);
  clear_txt();  

  // if I use this function, I can't wake it up with the M5 button anymore
  // M5.Axp.DeepSleep();

  // this works
  esp_deep_sleep_start();
}

Right now it does nothing, it only prints the battery level and goes to sleep after 10 seconds.
If I unplug the device when fully charged (4.1V), then press the "M5" button once in a while (less than once an hour), I noticed a battery life of about 12 hours, which is way too little.
I imagine that if I actually make it do stuff (like connecting to WiFi and perform a HTTP request), it would last even less.

Is this correct? Can I improve it somehow?

Thanks