As the brain gradually slips into unconsciousness it loses control over the muscles, says Dr John Shneerson, director of The Sleep Centre at Papworth Hospital in Cambridge, England.

In this relaxed state, muscles spasm in reaction to thoughts passing through the mind. These sometimes take the form of loud noises or a feeling of falling that can seem very real and may even wake you up. But "sleep starts," as the spasms are often called, are completely harmless.