In 1953 Nathaniel Kleitman and Eugene Aserinsky noticed sleepers’ eyes darting beneath closed lids and found that people woken in those windows reported vivid dreams. EEG later showed REM sleep looks like wakefulness: fast, desynchronized signals, theta rhythms in the hippocampus and gamma waves of 40 to 60 hertz. Acetylcholine floods the brain while histamine, serotonin and norepinephrine drop to zero, and Hobson and McCarley described brainstem REM-on and REM-off neurons switching the state. To keep the body still, the medulla hyperpolarizes motor neurons by 2 to 10 millivolts, producing paralysis below the neck. When that fails, REM behavior disorder lets people act out dreams, and in narcolepsy the switch intrudes into waking life as cataplexy. REM also suspends temperature control, so sleepers stop shivering, and cold enough rooms block REM entirely.

PGO waves fire from the brainstem into the visual cortex about every six seconds and drive the eye movements, while emotional regions light up and the prefrontal cortex dims. People woken from REM solve anagrams better, and SSRIs suppress REM until a vivid rebound. REM-like states appear in bearded dragons and jumping spiders, predators get more than prey, and newborns spend 80 percent of sleep in REM. Explanations range from memory consolidation and Crick and Mitchison’s unlearning of parasitic modes to tonic immobility and oxygenating the cornea.

• Jumping spiders whose legs curl during REM-like bouts, a sign of lost internal fluid pressure
• The ontogenetic hypothesis that REM wires the developing brain before birth
• Rats on flower pot platforms in water that fell in whenever REM paralysis began and died within four to six weeks
• The paradox that REM deprivation temporarily eases depressive symptoms
• Why humans on long term SSRIs survive suppressed REM while deprived rats do not