In extreme cases sleep disorders could be life-threatening

‘Perfectly sane by day, but transformed into disturbed alien creatures by night.’ In 2003, the Observer investigated, not werewolves, but parasomniacs.

Believed in the ancient world to signal demonic possession or mental illness, parasomnia makes sufferers behave in ways they never would when awake: they ‘can wake up covered in bruises and blood, but with little or no memory of their actions or details of the terror’.