Why the code snippet gets stuck when optimization is enabled?

There are two question about the code snippet below.

1. When the macro(i.e. NO_STUCK_WITH_OPTIMIZATION ) is not enabled, why this code snippet gets stuck when the optimization is enabled(i.e. -O1, -O2 or -O3) whereas the program works well if the optimization is not enabled?

2. And why the program no longer get stuck if std::this_thread::sleep_for() is added?

    #include <functional>
    #include <thread>
    #include <iostream>
    #include <chrono>
    #include <atomic>
   
    #ifdef NO_STUCK_WITH_OPTIMIZATION 
    using TYPE = std::atomic<int>;
    #else
    using TYPE = int;  //The progrom gets stuck if the optimization is enabled.
    #endif
   
    int main()
    {
        TYPE is_run{1};
        auto thread = std::thread([&is_run](){while(1==is_run){
              //std::this_thread::sleep_for(std::chrono::milliseconds(10)); //If this line is added, the program is no longer gets stuck. Why?
                                            }
            std::cout << "thread game over" << std::endl;
        });
   
        std::this_thread::sleep_for(std::chrono::seconds(1));
        is_run = 0;
        thread.join();
    }