The Old Hag is one of many of the global names assigned to the very real "Sleep Paralyzsis" phenomema.

One awakens to see a dark figure watching near them.  One can not breathe or move.  This may manifest in that the apparition is on their chest, holding them down and preventing them from breathing.  There have been incidents of supposed violent attacks by the apparition with occasional real injuries resulting from striking nearby objects trying to fend off the illusionary attack the instant they are able to move and breathe again.

Though science has detailed and rational explanations for this, this dark being has been attributed to being a Succubi or Succubus, a demon, or a ghost.  Saying a prayer, silently if one can do nothing else, has been attributed to warding away the apparition.  Once the apparition vanishes, one is able to breathe and move again. There has been some speculation that some incidents of this have contributed to Sudden Unexpected Death Syndrome.