Use a state machine instead:

bool  flashing;
unsigned long lastflashingChange;

void loop() { 
    unsigned long currentMillis = millis;
    if(flashing){
        flash();  //update flash state and change IO pin
        if (lastflashingChange - currentMillis >= awake_interval) {  //awake_interval = 250 ms    
            flahsing = false;
            //turn off the led
            lastflashingChange = currentMillis;
        }
    } else {
        if (lastflashingChange - currentMillis >= sleep_interval) {  //awake_interval = 250 ms    
            flashing = true; 
            //possible initialize flash state
            lastflashingChange = currentMillis;
        }
    }
 
    previousMillis = millis();
}

And a similar bit of code inside flash()

bool flashOn;
unsigned long lastFlashLightChange;
void flash() {
    unsigned long x = millis();

    if(flashOn){
        if (lastFlashLightChange - x >= t1) {
            flashOn = false;
            lights_off();
            lastFlashLightChange = currentMillis;
        }
    } else {
        if (lastFlashLightChange - x >= t2) {
            flashOn = true;
            lights_on();
            lastFlashLightChange = currentMillis;
        }
    }

}

Use a state machine instead:

bool  flashing;
unsigned long lastflashingChange;

void loop() { 
    unsigned long currentMillis = millis;
    if(flashing){
        flash();  //update flash state and change IO pin
        if (lastflashingChange - currentMillis >= awake_interval) {  //awake_interval = 250 ms    
            flahsing = false;
            //turn off the led
            lastflashingChange = currentMillis;
        }
    } else {
        if (lastflashingChange - currentMillis >= sleep_interval) {  //awake_interval = 250 ms    
            flashing = true; 
            //possible initialize flash state
            lastflashingChange = currentMillis;
        }
    }
}

And a similar bit of code inside flash()

bool flashOn;
unsigned long lastFlashLightChange;
void flash() {
    unsigned long x = millis();

    if(flashOn){
        if (lastFlashLightChange - x >= t1) {
            flashOn = false;
            lights_off();
            lastFlashLightChange = currentMillis;
        }
    } else {
        if (lastFlashLightChange - x >= t2) {
            flashOn = true;
            lights_on();
            lastFlashLightChange = currentMillis;
        }
    }

}