The old hag

Everyone has a name for this

The strongest argument that this is one phenomenon rather than many is that it has been independently named everywhere, by people with no way to compare notes. The English word nightmare carries the evidence in it: the mare was a being that sat on the chest of a sleeper, and the word is older than any modern account of it.

What is explained, and what is not

Sleep paralysis explains a great deal of this and it should be said first. The body wakes before the switch that keeps you still during dreaming flips back. The immobility is real and it has a mechanism. The sense of pressure on the chest has a plausible mechanism too, in how breathing feels when the muscles you would normally use are still switched off.

What the label does not settle is why the room is so often occupied, and occupied by something so specific. The research suggests the intruder takes a shape the sleeper's culture already supplies, which explains the jinn in Egypt and the Pandafeche in Italy rather neatly. It explains less well why the shape is so often an old woman, or a figure standing rather than lying, in places with no such story to draw on.

It is worth knowing that the medical explanation exists and that it is good. It is also worth knowing that it was never designed to answer the question most people actually have, which is why the thing in the room keeps being the same thing.