How to call functions asynchronously in C ++?

I tried std::async with all its parameters that I was able to google. Functions are still performed sequentially. Functions are called in an infinite loop in main (). In one of the called functions, there is a for loop containing a Sleep. In general, all the functions that contain Sleep, break others, just some make it less noticeable. When you run this function, others start to run very rarely. All functions of type void, return nothing. Please tell me how to do it right?