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Why do some sleep disorders turn normal dreams into terrifying nightmares? And what do they tell us about the workings of the brain?

Dreaming usually occurs in REM sleep (Rapid Eye Movement sleep) when our brains are very active, but our bodies are almost completely paralysed. But sometimes, the switch that paralyses our muscles is faulty, causing conditions that can significantly impact our days and nights.

Neurologist, Dr Guy Leschziner, from Guy's and St Thomas' hospitals in London, introduces us to some of his patients with problems that include sleep paralysis and hallucinations.

We meet Evelyn whose sleep has been plagued by visions so scary, she doesn鈥檛 want to go to bed; we hear from Christian who has narcolepsy, a rare brain condition that makes him suddenly fall asleep at inappropriate times. And we meet John who has REM sleep behaviour disorder (known as RBD) which causes him violently to act out his dreams, sometimes injuring himself and his wife, Liz. And we learn how RBD may be an early warning of degenerative brain conditions like Parkinson's disease and certain types of dementia.

These disorders arising from dreaming sleep help us to understand why we dream and the role of REM sleep.

(Photo: Sleeping woman having nightmare, Credit: Getty Images)