Editing Sleep paralysis (section)

=== Threat hyper-vigilance ===
A [[Hypervigilance|hyper-vigilant]] state created in the [[midbrain]] may further contribute to hallucinations.<ref name="Cheyneninenine" /> More specifically, the emergency response is activated in the brain when individuals wake up paralyzed and feel vulnerable to attack. This helplessness can intensify the effects of the threat response well above the level typical of normal dreams, which could explain why such visions during sleep paralysis are so vivid. The threat-activated vigilance system is a protective mechanism that differentiates between dangerous situations and determines whether the fear response is appropriate.<ref name="Cheynetwothree" />

The hyper-vigilance response can lead to the creation of endogenous stimuli that contribute to the perceived threat.<ref name=Cheyneninenine /> A similar process may explain hallucinations, with slight variations, in which an evil presence is perceived by the subject to be attempting to suffocate them, either by pressing heavily on the chest or by strangulation. A neurological explanation holds that this results from a combination of the threat vigilance activation system and the muscle paralysis associated with sleep paralysis that removes voluntary control of breathing. Several features of REM breathing patterns exacerbate the feeling of suffocation.<ref name=Cheynetwothree /> These include shallow rapid breathing, [[hypercapnia]], and slight blockage of the airway, which is a symptom prevalent in [[sleep apnea]] patients.<ref name=Cheyneninenine />

According to this account, the subjects attempt to breathe deeply and find themselves unable to do so, creating a sensation of resistance, which the threat-activated vigilance system interprets as an unearthly being sitting on their chest, threatening suffocation.<ref name=Cheyneninenine /> The sensation of entrapment causes a feedback loop when the fear of suffocation increases as a result of continued helplessness, causing the subjects to struggle to end the SP episode.<ref name=Cheynetwothree />