The postman always rings twice and when he does, this excited pooch is sent instantly to sleep. Mable, the five-year-old bearded collie cross, suffers from canine narcolepsy, a condition that is so rare vets have been left scratching their heads over how to treat her. When she has one of her episodes, she wags her tail furiously and runs around yelping before she hits the ground, "like she's been struck by a bullet," according to owner, Trevor Gliddon, 46. The sleep attacks are brought on by situations where Mable becomes excited, such as the daily arrival of the postman, playing with other dogs, or if she is petted too vigourously. (Footage by Barcroft Media/Getty Images)