Is it ok to use "async" with a ThreadStart method?

I have a Windows Service that uses Thread and SemaphoreSlim to perform some "work" every 60 seconds.

class Daemon
{
    private SemaphoreSlim _semaphore;
    private Thread _thread;

    public void Stop()
    {
        _semaphore.Release();
        _thread.Join();
    }

    public void Start()
    {
        _semaphore = new SemaphoreSlim(0);
        _thread = new Thread(DoWork);
        _thread.Start();
    }

    private void DoWork()
    {
        while (true)
        {
            // Do some work here

            // Wait for 60 seconds, or exit if the Semaphore is released
            if (_semaphore.Wait(60 * 1000))                
            {
                return;
            }
        }
    }
}

I'd like to call an asynchronous method from DoWork. In order to use the await keyword I must add async to DoWork:

private async void DoWork()

1. Is there any reason not to do this?
2. Is DoWork actually able to run asynchronously, if it's already running inside a dedicated thread?