The image moves or does not move based on very small movements of the eyes. If one is rested and not stressed then the eyes focus and hold focus more precisely and with little or no effort. As one is more weary from lack of sleep or stress, the focus of the eyes become less stable and requires more effort to maintain and there are involuntary micro movements as you strain to keep focus. This micro movement causes slight focusing adjustments and movement of the eye that are not noticed when looking at normal everyday objects and scenes, but become very apparent while looking at this image due to the design.

You, sir, are tired.