| Some deaf people have been known to continue signing even after they drift into sleep, through unconscious movements of the hands. The phenomenon is similar to talking in your sleep: The brain's language and motor circuits remain active while you snooze, though they are suppressed, and during sleep-talking episodes, brain activity more closely resembles the awake state, allowing speech to slip out. Sign and spoken languages involve similar neural processes, so while hearing people may talk in their sleep, this subconscious impulse takes the form of sleep signing or involuntary hand movements for deaf people. Sleep signing has been documented since as early as 1935, when electrophysiology studies found deaf people made signing motions while asleep. Bursts of activity were observed in the fingers and arms, a pattern not seen in hearing sleepers. In one widely cited 2017 case, a 71-year-old deaf man with REM sleep behavior disorder was observed signing fluently in his sleep. Because that disorder prevents the usual temporary paralysis that occurs during REM (rapid eye movement) sleep, he was able to sign so clearly that researchers could even decode aspects of his dreams. This same mind-body link may also explain why dogs sometimes bark or twitch their paws during REM sleep and why some chimpanzees who know sign language have also been observed signing as they snooze. | 
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