One moment, your dog is barking at the door. The next, she's collapsed on her side, sleeping.
This condition is called narcolepsy, and according to Stanford University researchers, it's caused by a mutation in the hypocretin receptor 2 gene that regulates sleep.
The condition is relatively common among dogs, especially Labrador retrievers, poodles, dachshunds and Doberman pinschers. Narcoleptic episodes can last from a few seconds to half an hour.
When a narcoleptic episode happens, the dog will fall unconscious on its side or stomach. It usually happens during periods of excitement, such as going to the park or seeing someone at the door.
A visit to the vet might be warranted, but as long as you manage your dog's environment so she doesn't get too excited, there should be no long-term ill effects from the condition.