Up to half of all people will wake at least once unable to move, with crushing pressure on the chest and a dark figure in the room. This episode explains sleep paralysis as an overlap of REM sleep and waking: serotonin, the wake signal, and acetylcholine, which drives REM, fire at the same time, so the mind is alert while REM atonia keeps the body locked. Dream activity bleeds into perception as static, buzzing, whispers, roars and electric tingles. The midbrain’s threat vigilance system reacts to the helplessness, and because REM breathing is shallow and cannot be deepened at will, the brain reads that resistance as something sitting on the chest. Vestibular glitches add floating, spinning and out of body sensations.

Cultures have named the intruder for centuries. The word nightmare comes from the Old English mare, a goblin that sat on sleepers’ chests. In Newfoundland the Old Hag was fought with a hag board, a shingle studded with upturned nails strapped to the chest, and in Italy the Pandafeche witch was distracted by a bag of sand or beans she must count. Japan calls it kanashibari, Cambodia says the ghost pushes you down, Scrooge blamed Marley on an undigested bit of beef, and many Americans describe alien abductions. A comparison of Egypt, where episodes are attributed to the jinn, with Denmark found episodes three times as frequent and longer in Egypt.

• Why the brain invents an intruder to explain paralysis and restricted breathing
• Half of Egyptian sufferers fear dying of suffocation during an episode, versus 17 percent in Denmark
• Sleeping on the back, sleep deprivation and shift work as the triggers researchers cite
• The difference between isolated sleep paralysis on waking and the episodes at sleep onset seen in narcolepsy
• CBT-ISP, which teaches diaphragmatic breathing and reframing, and the use of tricyclics or SSRIs in severe cases