Old Hag Syndrome

Why the chest

The paralysis of REM sleep persists into waking, which removes voluntary control of breathing. REM breathing is shallow and rapid, produces hypercapnia, and involves slight blockage of the airway, the same blockage that is pronounced in sleep apnoea.

A person in that state tries to breathe deeply and cannot. What they feel is resistance. The threat-activated vigilance system, already running because they have woken paralysed and defenceless, interprets the resistance as something heavy on the chest.

The feedback loop follows and is the reason episodes feel long. Fear of suffocation increases with continued helplessness, which drives the struggle to end the episode, which sustains the fear. Episodes generally last between one and six minutes.

Why a hostile presence appears

Waking paralysed and vulnerable activates the emergency response in the midbrain. The threat-activated vigilance system is a protective mechanism that decides whether a fear response is warranted, and in an unresolvable situation it can amplify well beyond the level of an ordinary dream, which is offered as the reason these visions are so vivid.

Crucially, the hyper-vigilant state can generate its own stimuli. Having decided a threat exists, the system produces the evidence for one, and an intruder appears.

The hallucinations sort into three types with a measured relationship. The belief that an intruder is present and the incubus, the entity pressing down, correlate highly with each other, and both correlate moderately with the third, vestibular-motor disorientation or the sensation of floating out of the body. The third is different in kind, since it does not involve the threat vigilance system at all: a neurological hypothesis attributes it to a brief myoclonic spike in the cerebellum, which normally supplies information about body position.

The sounds reported divide in two:

• humming, hissing, static, zapping and buzzing
• voices, whispers, screaming, growling and roars Feelings of drowning, sinking, being dragged out of bed, flying, numbness and electric tingling are all documented.

What the mechanism underneath is

Sleep paralysis is a parasomnia arising from a dysfunctional overlap of REM and waking. Polysomnographic studies find shorter REM latency than normal, shortened sleep cycles, and fragmentation of REM sleep, which is exactly what disrupted sleep patterns produce.

The neurochemical account has the sleep-regulating populations out of balance: cholinergic sleep-on populations hyperactivated and serotonergic sleep-off populations under-activated, so the signals that would produce full arousal cannot overcome the signals holding the brain asleep.

One consequence explains why the experience feels so external. During normal REM sleep the threshold for a stimulus to cause arousal is greatly raised. In people reporting sleep paralysis there is almost no blocking of external stimuli, so the room comes through while the body does not respond.

The paralysis is not total. Electro-oculography shows eye movement remains possible. Speech does not.

How many people experience it

Between eight and fifty per cent of people experience it at some point, which is a range wide enough to reflect how the question was asked. About five per cent have regular episodes. Men and women are affected equally.

Twin studies find that where one identical twin experiences sleep paralysis the other is very likely to as well, which points to a physiological disruption without yet identifying which one. Risk rises with insomnia, sleep deprivation, an erratic schedule, stress and physical fatigue.

Where episodes occur without narcolepsy or another cause they are called isolated sleep paralysis, and where they are frequent enough to cause distress, recurrent isolated sleep paralysis. The Munich Parasomnia Screening is among the instruments used.

Which conditions are ruled out

Cataplexy from narcolepsy is physically indistinguishable, and the material gives a practical way to tell them apart: narcoleptic attacks are more common while falling asleep, and isolated sleep paralysis more common on waking.

Exploding head syndrome produces brief loud hallucinations without paralysis. Sleep terrors are not REM-based and involve no awareness of surroundings. Nocturnal panic attacks have the fear and distress without the paralysis or the dream imagery.

Where it goes when nobody explains it

Sleep paralysis is believed to have played a role in the creation of stories about alien abduction and other paranormal events, which is the same experience filed under a modern rather than a folkloric explanation.

Treatment has been poorly studied. The first recommendation in the material is not a treatment at all: that people be reassured the condition is common and generally not serious. For an experience this frightening and this thoroughly explained, that may be the most effective thing available.