Lady Omega

Fatigue, among other things, can cause one of the rarest and most fascinating hallucinations known: that of the doppelganger. A person who has this experience sees his mirror image, usually facing him from three or four feet away. It reproduces the viewer’s facial expressions, posture, and movements as though reflected in a mirror. The image is said to be transparent, like a slide or movie projected on glass. In some cultures these hallucinations have been interpreted as visitations from the person’s soul or as premonitions of death.

This hallucination, which usually occurs early in the morning or late at night and lasts only a matter of seconds, can appear to normal minds under stress or fatigue. However, it is more common among people with serious disorders such as epilepsy, brain lesions, migraine headaches, and states of delirium.

In particularly bizarre cases, the person may see the double standing in an adjacent room. Others have reported seeing this kind of apparition standing behind their own reflection in the mirror.
Source