The Old Hag is a witch-like night visitor whose ominous presence
terrorises individuals who are half-awake and unable to move. Similar, but even
more ancient, legendary beings are the Mara and the Alp. Both
beings are said to sit on their victims, but while Mara bring only bad dreams
(hence the term 'nightmare') the Alp can actually crush the life
from ill-fated snoozers and the occasional farm animal.
Modern science would explain all such incidents as a result of sleep paralysis,
a state in which a person is conscious but unable to move and experiences the
sensation of being watched.
The modern version of The Old Hag are Aliens.