The sexual attack happened after the 48-year-old got up and walked out of the house and down the road. She later told detectives that she had been accosted and raped during the incident, Daily Mail reported.
Police are investigating the claims and said officers from Operation Sapphire, the Metropolitan Police's specialist rape investigation, were involved in the probe.
The victim, understood to be a child carer, had been at a dinner party with friends at a house in north London. It is believed she decided to spend the night along with other guests, and had gone to bed around midnight.
A source, who requested not being named, said the woman then reportedly left the house and was raped while out on the road.
"She suffers from somnambulism which means that she sometimes gets up and sleep-walks in the middle of the night," the source added.