Game loop not functioning correctly

I'm working on a game loop in C++ using OpenGL and GLFW and it works like expected (I think), except when I'm focused in the window the FPS drops to 1fps (or 1000ms per frame) but when I'm not focused in the window the FPS is ~240fps. I think it has something to do with the timer, but I'm not quote sure why it would.

I'm running the program on OS X Yosemite 10.10.2 and building it with Xcode.

The debug output of the FPS:

333.333333 ms/frame (3 fps)
1000.000000 ms/frame (1 fps)
4.694836 ms/frame (213 fps)
4.149378 ms/frame (241 fps)
4.166667 ms/frame (240 fps)
4.166667 ms/frame (240 fps)
4.149378 ms/frame (241 fps)
4.149378 ms/frame (241 fps)
142.857143 ms/frame (7 fps)
1000.000000 ms/frame (1 fps)
4.716981 ms/frame (212 fps)

Here is the game loop code:

double unprocessedTime = 0;
double frameTime = (double)1 / (double)240;
double lastTime = glfwGetTime();
double frameCounter = 0;

int frames = 0;

bool isRunning = true;

while(isRunning)
{
    bool shouldRender = false;

    double startTime = glfwGetTime();
    double delta = startTime - lastTime;

    unprocessedTime += delta;
    frameCounter += delta;
    lastTime = startTime;

    while(unprocessedTime >= frameTime)
    {
        unprocessedTime -= frameTime;
        shouldRender = true;

        if(window.ShouldWindowClose())
        {
            isRunning = false;
        }

        game.Tick();

        if(frameCounter >= 1)
        {
            printf("%f ms/frame (%d fps)\n", ((double)1000 / frames), frames);

            frameCounter = 0;
            frames = 0;
        }
    }

    if(shouldRender)
    {
        game.Render();

        frames++;
    }
    else
    {
        usleep(1000);
    }
}