...

private boolean running;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    running = true;
    new Thread(null, work, "myThread").start();
}

Runnable work = new Runnable() {

    @Override
    public void run() {
        while (running) {
            //Doing work
        }
    }
};

@Override
protected void onPause() {
    running = false;
    super.onPause();
}

@Override
protected void onResume(){
    running = true;
    super.onResume();
}

Runnable work = new Runnable() {

    @Override
    public void run() {
        while () {
          if(running){
            //Doing work
          }
        }
    }
};